Reflected in all aspects of the tax law. The tax law not only applies fairly to all taxpayers, but also applies fairly between taxpayers and taxpayers. Tax fairness is not only reflected in horizontal fairness, but also in vertical fairness. It is not only formal fairness, but also substantive fairness. Judging from various types of tax legal relations [10], the principle of fair priority also permeates them. The relationship between tax creditor's rights and debts (the legal relationship of tax collection and payment) itself is complementary to the principle of fairness. The tax law not only requires the tax authorities to treat all taxpayers fairly, but also treats both tax collectors fairly. The tax administration law gives the tax authorities the right to collect and manage, but also limits these rights, and gives taxpayers a series of rights such as the right to know, the right to defend representations, the right to sue and report, and the right to obtain relief to balance the rights of both parties. The tax relief law is biased towards the taxpayer's right relief in order to realize the substantive fair treatment of both parties. In the relationship between tax creditor's rights and debts, fairness is always given priority, and it is never allowed to sacrifice fairness for tax efficiency. In the legal relationship of tax administrative litigation, the law requires fair (more precisely, substantive fairness) treatment of both parties. In the higher-level tax legal relationship, that is, the constitutional tax legal relationship and the international tax legal relationship, the principle of giving priority to fairness and giving consideration to efficiency of tax law can be more clearly reflected. The following is a detailed analysis of the two. The constitutional legal relationship of taxation refers to the tax legal relationship between taxpayers (people) and the state according to the provisions of the Constitution on taxation [1 1]. Rousseau mentioned in his "On Social Contract" that people living under natural law need to find a way to combine in order to end the law of the jungle situation in which life, freedom and property can only be preserved by strength. So that it can defend and protect the person and wealth of each partner with all its strength, and because of this combination, every individual who is United with all is just obeying himself and still can be as free as before. As a result, the people produced this combination by concluding a social contract-the people are the same body (that is, the sovereign). Under the social contract, the sovereign is the representative of the general will and enjoys the authority to formulate the constitution to express the general will. The government is only the executor of the sovereign, responsible for enforcing the law and maintaining social and political freedom [12]. If we understand the country in the constitutional legal relationship of taxation as a sovereign, then the sovereign who represents the general will must strictly abide by the concept of fairness and priority when stipulating taxation in the constitution, because the general will is first reflected in the requirements of fairness and equality; If the country in the constitutional legal relationship of taxation is understood as the government, then the government, which is only the executor of the sovereign, naturally cannot violate the public will's requirement of fairness and priority. Some scholars also believe that from the perspective of the process of the emergence of the state and the law, everyone ends the state of fighting with each other to conclude a contract to produce the state, and the state and the people conclude a contract to produce the constitution and its concretization-law (including tax law or legal provisions on taxation) [13]. In other words, the provisions on taxation in the constitution and their concretization (tax law) can be regarded as a contract signed between the state and taxpayers (people). Taxpayers (people) pay taxes according to the contract to buy public products and services, and the state provides public products and services according to the contract. Since the tax law is just such a contract, the principle of fair priority must be its reasonable core. People are willing to give up part of their rights to establish a country and pay taxes to maintain its operation because they believe that this country is good, operates for the overall interests of the whole society and ultimately benefits themselves. Therefore, the tax law must "fairly" collect taxes from people to maintain this trust. The constitutional legal relationship of taxation strictly carries out the spirit of contract and the principle of equality, and reflects the nature of creditor's rights and debts of the tax legal relationship and the most fundamental "fair value" of the tax law most profoundly among all the tax legal relationships. International tax legal relations include international tax creditor's rights and debts legal relations and international tax distribution legal relations. The former is only an extension of the relationship between creditor's rights and debts in domestic tax law in international tax law, and naturally embodies the principle of giving priority to fairness and giving consideration to efficiency. The latter refers to the legal relationship arising from the distribution of international tax benefits between different sovereign countries. Although there is efficiency value in international tax law, it is far less important than fair value, especially in the legal relationship of tax distribution between countries. According to Article 10 of the Charter of Economic Rights and Duties of States [14], the legal relationship of international tax distribution embodies the principle of fair distribution of international tax law, that is, sovereign countries participate in the coordination process of tax benefit distribution among themselves on the basis of independence of their tax jurisdictions, and finally achieve the result of taking care of the interests of all parties and being relatively fair. Its essence is fairness and mutual benefit. The principle of fair distribution in international tax law is an excellent embodiment of fairness first and efficiency second. This can be seen in the activities of unifying the tax jurisdiction of various countries. There are generally "resident jurisdiction" and "income source jurisdiction" in tax jurisdiction of various countries, and countries generally choose to apply or give priority to one of them, but now the voice of unifying international tax jurisdiction as income source jurisdiction is getting higher and higher. Unification is more about efficiency, while unification in the jurisdiction of the source of income is more about fairness. The legal basis for implementing the jurisdiction of a single source of income is the principle of giving priority to fairness and giving consideration to efficiency in tax law. If efficiency is given priority, countries with economic and technological advantages are allowed to levy taxes on transnational taxable objects in an efficient way, and other countries with relative disadvantages are deprived of the right to levy taxes on the same object on the grounds that they have to pay higher tax costs than the former, which will inevitably undermine the fair state among countries in the field of international tax distribution and violate the basic principles of fairness and mutual benefit in international economic exchanges [15]. Moreover, Due to the great difference in the level of economic development among countries, developed countries have obvious advantages over developing countries in the international market competition, which is "formal fairness and substantive unfairness" for developing countries. The parallel situation of residents' tax jurisdiction and income source jurisdiction just deepens this unfairness. In the international investment market, developed countries are the majority owners of world capital, and the flow of capital, technology, materials and information is basically one-way. Developed countries invest a lot abroad, while developing countries mainly attract foreign investment. Residents' tax jurisdiction enables developed countries to exercise the right to tax domestic and foreign income, while developing countries actually only exercise the right to tax domestic income, which makes the international distribution of tax rights tilt towards developed countries, resulting in substantial unfairness in international competition [16]. From the perspective of the principle of tax opportunity, the tax burden should also be shared according to the opportunity of taxpayers to obtain income. Residents of developed countries invest in developing countries and make profits. Developing countries provide a decisive opportunity for them to make profits. If residents' tax jurisdiction is used, the efforts and sacrifices made by developing countries will gain nothing, which is obviously unfair. While exercising the tax jurisdiction of the source of income can achieve real fairness between multinational taxpayers and domestic taxpayers in international investment or international competition [17]. In addition, from the perspective of tax collection cost, the cost of income tax is the highest, followed by value-added tax, and the business tax is the lowest. Therefore, tax design, which adopts business tax as the main body, followed by value-added tax and supplemented by income tax, should be the most efficient tax collection. However, from the perspective of tax fairness, business tax is the most unfair, because it can't well embody the principle of capacity-based taxation, that is, no matter whether the taxpayer has income or not, as long as there is turnover, even if he is operating at a loss, he still has to pay business tax. The income tax law best embodies the principle of tax fairness. It not only strictly abides by the principle of taxation only when there is income, but also adopts a progressive tax rate on personal income tax to realize the vertical fairness of tax law. Therefore, tax design, an advanced country, puts income tax as the main tax category, and abandons or "ignores" the business tax system. China is also moving from turnover tax to income tax. To improve the tax system, we must give priority to efficiency and give consideration to fairness. It is not feasible to talk about fairness without efficiency and to talk about efficiency without fairness. Putting aside the efficient allocation of resources and the effective operation of economic mechanism to blindly pursue "fairness" will inevitably lead to the loss and waste of limited economic resources and the disorder of social and economic life. The tax reform is based on the theory of market economy and embodies the ideological policy of giving priority to efficiency and giving consideration to fairness, which can be said to be a milestone in China's tax reform.