In order to overcome the problem of speed overshoot and improve the anti-disturbance ability of PI control in the permanent magnet synchronous motor (PMSM) speed control system, a novel exponential reaching law with strong adaptive ability is proposed. This novel reaching law, based on the traditional exponential reaching law, improves the isokinetic term coefficient to a time-varying term, which can make the system converge to a given value faster, and the problem of the slow convergence speed of the traditional exponential reaching law is solved. In addition, the saturation function of the variable boundary layer is used to replace the traditional switching function to weaken the sliding mode chattering phenomenon. Lyapunov stability theory is used to analyze the stability of the novel exponential reaching law, and the speed loop sliding mode controller is designed based on the novel law. The designed controller is compared with PI control by MATLAB modeling and simulation. The simulation results show that the speed loop sliding mode controller based on the novel law can effectively improve the robustness of the PMSM speed control system.