There is a problem of weak dynamic performance caused by the dynamic coupling and back EMF terms in the torque closed-loop control system of a permanent magnet synchronous motor. Thus a sliding mode compensation control method based on a new voltage control law is proposed. First, based on the permanent magnet synchronous motor model, a new voltage control law is established to ensure the stability of the torque closed-loop control system. Second, using the functional relationship between the velocity estimation and the sliding mode surface, an adaptive velocity observer is designed to compensate the coupling and back-EMF terms of the d-axis and q-axis, and realize the speed closed-loop of the system at the same time. After that, a sliding mode surface with a sigmoid nonlinear smooth function is introduced, and a sliding mode compensation control strategy is constructed to realize the robustness of the control law to parameter and external perturbations during the entire motion process. Finally, a Matlab/Simulink simulation model and experimental platform of a 400 W permanent magnet synchronous motor controlled by TMS320F28335 are built. Simulation and experimental results show that the proposed control method has the characteristics of small torque and current fluctuation, fast response and strong robustness.