The actual operating conditions of electric vehicles were complicated, and the speed range and torque requirements of PMSM were higher. In order to improve the speed range of the builtin permanent magnet synchronous motor, the voltage closedloop feedback field weakening control method was studied and realized. The method used the voltage utilization as a reference to construct the voltage loop, and the weak feedback was calculated by the voltage feedback signal and the command signal. Current idreference value, thus realizing the permanent magnet synchronous motor weak flux expansion. The simulation model of the field weakening control system of the builtin permanent magnet synchronous motor was established on the MATLAB/Simulink platform. The simulation results showed that the system had a wide speed range and the interval was smooth and had good response characteristics.