Abstract: As a kind of adjustable speed motor, PMSM is very sensitive to voltage sags, especially when the driving process is strict with the speed and torque of motor. In this paper, the basic principle of variable-frequency adjustable-speed PMSM is briefly introduced. The influence mechanism of voltage sag on variable-frequency adjustable-speed PMSM is analyzed, and a simulation model is built with MATLAB/Simulink to verify it. An experimental research platform is developed. Three domestic and foreign brands of variable-frequency drives are selected as the driving equipment of PWSM, and the change of speed and stator current of PMSM under different variable-frequency drives are researched. The results show that the voltage sag will cause variable-frequency adjustable-speed PMSM to lose synchronism within a short period, and make the stator current go through a change process similar to the motor speed. The effects on the operating performance of motor under different variable-frequency drives is significantly different, and the motor speed decreases by a maximum of 30% of the rated speed, which brought great challenges to the operation stability of motor.