Abstract: In this paper, a composite control strategy of improved pulse high-frequency voltage injection method combined with a novel fourth-order sliding mode observer with reaching rate is designed to improve the accuracy of sensorless control of permanent magnet synchronous motor in the full speed domain. In the zero and low speed stages, the second-order generalized integrator is used to extract the position error signal of the motor, and the improved fourth-order sliding mode observer is used to estimate the rotor position information in the medium and high speed stages. Finally, the weighted switching algorithm is used to combine the improved pulse high-frequency voltage injection method and the new reachin g rate fourth-order sliding mode observer. The simulation results show that the designed composite control strategy can achieve stable and efficient full speed domain operation of the motor under load start and sudden load.