In order to improve the robustness and control accuracy of permanent magnet synchronous motor (PMSM) and reduce the influence of external disturbance on the control, a sliding mode control method is proposed based on a new reaching law. The new reaching law solves the contradiction between sliding mode surface reaching time and the system chattering in the regular reaching law. Based on the traditional power approaching law, a piecewise power approaching law is proposed. A linear term is added after the second power term to better reduce chattering. A sliding mode disturbance observer is designed with the load torque and rotational speed as the observation objects. The observation results are fed back to the speed control loop to further improve the robustness. The simulation results verify the feasibility and effectiveness of the method.