In order to improve the control accuracy of servo system of AC permanent magnet synchronous motor (PMSM), a sliding mode variable structure control method based on proportional switching control strategy was proposed. Firstly, the servo system model was given. Then, the sliding mode controller was designed, and the proportional switching control input was obtained by selecting the appropriate sliding mode surface. The responses of time-invariant and time-varying systems were discussed separately, and the response was compared with that of PI control to analyze the advantages and disadvantages of the method. The research results showed that the designed method had high control accuracy, good robustness and rapidity.