This paper designs a power system for an electric car. Particular emphasis is placed on the choice of motors and batteries to create a complete powertrain. For example, in the first chapter, we analyze the advantages and disadvantages of the internal structure, working principle and use of the motor as the entry point, and finally determine the type of motor to be selected. In the second chapter, we combine the final design goals, carry out specific mathematical calculations, and determine the specific parameters of the required motor power, torque and so on. Furthermore, several different brands but the same type of motor were analyzed.