The thermal management system, which is used to improve driving safety and thermal comfort, is one of the most important systems in electric vehicles. In recent years, researchers have coupled the heat pump system and the battery cooling system to effectively improve the heating COP (Coefficient of Performance). Therefore an accurate dynamic model of thermal management system plays a key role in investigating system performance and optimal control strategies. In this paper, an electric vehicle thermal management system based on four-way valve heat pump system is designed. The moving boundary method is improved by considering the unsteady flow of the external fluid, and then a 13-order dynamic model of the thermal management system is established. Firstly, the control equations of evaporator, condenser and chiller are derived according to the principle of conservation, and then a dynamic model of thermal management system is established in Simulink. Secondly, the steady and transient state parameters of the internal and external fluid of the heat exchangers are obtained through Amesim, and the parameters are compared with the results of the Simulink model to verify the accuracy of the dynamic model. Finally, on the basis of dynamic model, the influence of compressor speed and air flow rate of the heat exchanger is analyzed. The results show that the established dynamic model can accurately describe the state of the thermal management system, and the order of the model is lower, which is conducive to the research and control of the system.