To address uncertainties in the speed loop of surface-mounted permanent magnet synchronous motors (PMSMs), such as load disturbances and parameter mismatches, an anti-disturbance sliding mode control strategy based on a novel reaching law is proposed. First, a new adaptive reaching law is formulated, in which an adaptive gain of the system sliding variables is introduced into the constant-speed term. This design resolves the traditional conflict between convergence speed and chattering in exponential reaching laws, reduces tracking errors, and avoids abrupt changes in reaching speed. Subsequently, a speed-loop sliding mode controller is designed by integrating the proposed reaching law with a nonsingular integral terminal sliding surface. To further enhance disturbance-rejection capability, an extended sliding mode disturbance observer is implemented finally to estimate and compensate for speed-loop uncertainties. Simulation and experimental results indicate that the proposed anti-disturbance sliding mode control strategy significantly improves the dynamic response and robustness of the motor speed control system.