In order to reduce the dynamic error in the permanent magnet synchronous motor (PMSM) speed control, a sliding mode control (SMC) method with optimal integral performance was proposed. In this method, the dynamic error in SMC was taken as the performance index. On this basis, the optimal switching function was established, and the optimal control theory was used to design the sliding mode controller. The SMC system designed by this method could speed up the process of the system state variable reaching the sliding surface by continuously changing the slope of the sliding surface, and greatly improve the robustness to parameter perturbation and external disturbance. The simulation results showed that the timevarying sliding surface control method had the advantages of no overshoot, rapidity and stability, and improved the robustness of the system.