In the foreign exchange market, the central bank influences the exchange rate changes by inquiring about the exchange rate changes and issuing statements, so as to achieve the effect of intervention, which is called the "signal effect" of foreign exchange market intervention. In doing so, the central bank hopes that the foreign exchange market can get a signal that the central bank's monetary policy will change, or the expected exchange rate will change, and so on. Generally speaking, the foreign exchange market always reacts after receiving these signals for the first time. But if the central bank often intervenes in the market with "signal effect", and these signals are not all straight, then it will have the effect of "wolf coming" in the market. Intervention in the foreign exchange market is actually a change in the central bank's monetary policy, which means that the central bank buys and sells foreign exchange in the foreign exchange market with reference, and at the same time allows the domestic money supply and interest rates to change in a direction conducive to achieving the intervention goal. For example, if the mark continues to depreciate in the foreign exchange market, the Bundesbank can throw foreign exchange in the market to buy the mark to support the exchange rate of the mark. With the decrease in the circulation of marks, the German money supply decreases, and interest rates are on the rise. People are willing to keep more marks in the foreign exchange market, so that the exchange rate of marks will rise. Generally speaking, this kind of intervention is very effective, at the cost that the established domestic monetary policy will be affected, and the central bank is willing to take it when it sees that its currency exchange rate deviates from the equilibrium price for a long time. Whether the central bank's intervention is effective is not judged by the number and quantity of money used by the central bank.