With the increasing shortage of oil supply and environmental pollution, electric vehicles powered by electric energy have become the focus of attention in the industry with its advantages of energy saving and environmental protection [1]. Since the 1980s, many developed countries have invested heavily in the research and development of electric vehicles, and China's "863 Plan" has clearly made electric vehicles a key research project. At present, the R&D level of electric vehicles in China is basically at the same starting line as that of developed countries, and even exceeds that of foreign countries in some aspects [2]. In 2005, the first generation of hybrid electric commercial vehicles in China passed the demonstration and acceptance [3]. France, Japan, the United States, Germany, etc. Pure electric vehicles with commercial level have been developed through experiments and demonstration operations, such as electric vehicles P 106 and Citroen AX of French PSA Company, RAV-4EV of Japanese Toyota Motor Company and EV 1 of American General Motors Company. China has also included the research and development of electric vehicles in the "Eighth Five-Year Plan" and "Ninth Five-Year Plan" national key scientific and technological projects, and in June, Shantou National Electric Vehicle Experimental Demonstration Base 1996 was built in Guangdong. During the Tenth Five-Year Plan period, the Ministry of Science and Technology listed the electric vehicle project as a national "863" major project. Beijing (led by Beijing Institute of Technology) not only subsidizes batteries, motors and their control systems, and vehicle control systems, but also mainly subsidizes the development and demonstration operation of pure electric buses. On June 2, 2005, with the official approval of the National Development and Reform Commission, 14 lead-acid battery pure electric buses were put into commercial operation on the Beijing bus line12 1. Another project funded Tianjin Qingyuan Power Company (China Automobile Technology Research Center) to carry out the research and development and demonstration operation of pure electric vehicles. Five pure electric vehicles were exported to the United States for the first time in early 2005 [4]. Although electric vehicle has many advantages, it can't replace the traditional gas power mode, and hybrid vehicle is the most promising model for industrialization and marketization among new clean power vehicles at present, and its development direction is fuel cell vehicle with real zero emission, no pollution and no fuel consumption. Now hybrid electric vehicles have been industrialized in Europe, America and Japan [3], but in China, they are still in their infancy and have not been industrialized. 2. Classification and Principle of Hybrid Electric Vehicle (HEV) A hybrid electric vehicle (HEV) is a vehicle that combines electric drive and auxiliary power (APU) to give full play to their respective advantages and combine them to produce advantages. The auxiliary power can be a prime Mover that burns a certain fuel, such as an internal combustion engine, a gas turbine or other generator sets. According to the different connection modes of hybrid power system, hybrid electric vehicles can be divided into three structural forms: series, parallel and series-parallel, each with its own advantages. 2. 1 series hybrid power system is shown in figure 1. The series structure is characterized by electrical recombination. The engine directly drives the generator to supply power to the energy storage device and the traction motor, which is used to drive the wheels. The energy storage device plays a regulating role between the engine output and the motor demand. Its advantage is that the operation of the engine is not affected by the speed and road conditions, and it is suitable for frequent starting, acceleration and low-speed operation of vehicles. The engine runs near the best working point, avoiding idle and low-speed conditions, thus improving efficiency and emission performance. However, in the process of conversion between mechanical energy and electric energy, there is efficiency loss, and it is difficult to achieve the purpose of obviously reducing fuel consumption. At present, it is mainly used in city buses, but rarely used in cars. 2.2 The schematic diagram of parallel-parallel hybrid power system is shown in Figure 2. The parallel structure is characterized by mechanical compounding. The engine is directly connected with the drive axle through the schematic device of the variable-speed parallel hybrid power system. The motor can be used as a motor or generator at the same time to balance the load on the engine and make it work in the high efficiency area. However, due to the mechanical connection between the engine and the drive axle, the engine can not run at the best working point under urban working conditions, and the fuel economy of the vehicle is worse than that of the series connection. Among them, the speed compound device is similar to the differential, and it is difficult to be adopted in practice because the output torque of the engine and the motor is always equal. The combination of mechanical power in the single-shaft torque compound vehicle drive system is realized at the output end of the engine crankshaft, and the transmission is a single-shaft input, which belongs to Honda Insight. The combination of two-axis torque combined with mechanical power is realized on the output shaft of the transmission. The engine and motor adopt different transmission systems, and the transmission is double-ended input. The hybrid power system designed by Warsaw University of Technology belongs to this shape, and this structure can also realize stepless speed change, but it cannot realize the direct superposition of engine output torque and motor output torque. In the traction compound system, the combination of mechanical power is realized on the driving wheel through the road surface. With two independent drive systems, it can realize all-wheel drive, which is mainly suitable for SUV. Toyota's THS-C system belongs to this form.