Abstract: To solve the internal and external disturbances in permanent magnet synchronous motors（PMSM） with parameter variations, modeling error and load uncertainties, a reduced-order extended state observer（RESO） based on composite sliding mode control strategy was proposed. Based on the state space model, a novel RESO was designed to estimate and eliminate the internal and external disturbances with considering the speed signal of PMSM measured by the encoder, and the performance for the RESO was analyzed. The composite sliding mode control method was introduced to design the sliding mode surface and control law for improving the anti-disturbance ability of the whole control system, and the stability of the control strategy was analyzed by Lyapunov method. The hardware-in-the-loop speed control platform of PMSM was built based on the control core of DSP chip TMS320F28335 and the MATLAB control module. The proposed method was simulated and compared with traditional PI and linear active disturbance rejection control methods. The results show that the speed variation of the system with loading can be reduced more than 60% in contrast to traditional system with excellent performance of anti-disturbance and regulation speed.