the so-called system simulation is to establish a simulation model that can describe the system structure or behavior process and has a certain logical relationship or quantitative relationship based on the analysis of the nature of the system elements and their relationships according to the purpose of system analysis, and then conduct experiments or quantitative analysis to obtain all kinds of information needed for correct decision. System simulation is a new discipline gradually formed with the development of computer technology since the late 194s. Simulation is the process of establishing an actual system model and making experimental research on the actual system by using the seen model [2]. At first, the simulation technology was mainly used in a few fields, such as aviation, aerospace and atomic reactors, which were expensive, long-term, dangerous and difficult to realize the actual system test. Later, it gradually developed into some major industrial sectors, such as electric power, petroleum, chemical industry, metallurgy and machinery, and further expanded to some non-engineering system fields, such as social system, economic system, transportation system and ecosystem. It can be said that modern system simulation technology and comprehensive simulation system have become an indispensable means of analysis, research, design, evaluation, decision-making and training for any complex system, especially for high-tech industries. Its application scope is constantly expanding, and its application benefit is becoming more and more obvious.

1. System simulation and its classification

System simulation is a comprehensive experimental discipline, which is based on control theory, similarity theory, information processing technology and computer elementary theory, uses computers and other special physical effect equipment as tools, uses system models to test real or hypothetical systems, and analyzes the experimental results with the help of experts' empirical knowledge, statistical data and information materials, and then makes decisions. In a broad sense, the method of system simulation is applicable to any field, whether it is an engineering system (machinery, chemical industry, electric power, electronics, etc.) or a non-engineering system (transportation, management, economy, politics, etc.).

according to different models, system simulation can be divided into physical simulation, mathematical simulation and physical-mathematical simulation (semi-physical simulation); According to the types of computers, it can be divided into analog simulation, digital simulation and hybrid simulation; According to the characteristics of the system; It can be divided into continuous system simulation, discrete time system (sampling system) simulation and discrete event system simulation. According to the relationship between simulation clock and actual clock, it can be divided into real-time simulation, under-real-time simulation and ultra-real-time simulation.