In the case of high speed weak magnetic field or high overload requirements of the motor, it was necessary to improve the voltage utilization rate of DC bus through pulse width modulation (PWM) overmodulation operation. The sensorless permanent magnet synchronous motor (PMSM) vector control (field oriented control, FOC) technology based on flux observation had the advantages of simple algorithm and easy implementation, but the voltage limiting link in the case of overmodulation would cause the error of flux observation and rotor position angle estimation. In order to improve the control performance of the system under PWM overmodulation, the traditional FOC control block diagram was improved, which eliminated the voltage limiting link and caculated the actual voltage obtained from duty ratio to estimate the stator flux linkage. MATLAB simulation verified the effectiveness of this method. The results showed that the new method could make full use of the inherent limiting characteristics of PWM, which could not only simplify the control algorithm but also improve the accuracy of flux observation, and had good technical and economic performance.