Abstract: To address the challenge of sensorless control for Permanent Magnet Synchronous Motors (PMSM) across the full speed range, a mode-switching composite strategy based on an improved Sliding Mode Observer (SMO) and high-frequency injection is proposed. In the low-speed region, a high-frequency pulsating square-wave voltage injection method is adopted. In the medium-to-high speed region, an SMO based on the Sigmoid function and an improved exponential reaching law is constructed to suppress high-frequency chattering. Considering the limitations of individual algorithms, a weighted switching algorithm based on speed interval division is designed to achieve smooth transitions. Matlab/Simulink simulation results demonstrate that the estimated speed after weighted fusion closely tracks the actual speed across the entire speed range, exhibiting satisfactory dynamic response, which verifies the feasibility and effectiveness of the proposed composite control strategy.