To address the insufficient adaptability of single-strategy control in the full-speed range operation of permanent magnet synchronous motors (PMSMs), this paper proposes a composite control strategy for PMSM operation across the entire speed range. By integrating IF (Indirect Field-Oriented) control with a high-order sliding mode observer (HOSMO), high-performance control from zero speed to medium-high speeds is achieved. In the low-speed range, a forced IF starting scheme is employed, combined with open-loop speed control and closed-loop current control, ensuring smooth motor startup. During medium-high-speed operation, a high-order sliding mode observer combined with a phase-locked loop (PLL) is utilized to accurately estimate rotor position information. In the transition region between speed ranges, an appropriate switching strategy is applied to achieve seamless transition from low to medium-high speeds. Finally, simulation experiments are conducted to validate the effectiveness and reliability of the proposed control strategy.