<p>In this paper, an active disturbance rejection control (ADRC) strategy with a decoupled high-order extended state observer (DHESO) is proposed to suppress periodic disturbances for permanent magnet synchronous motor (PMSM) servo systems. By comprehensive consideration of disturbance rejection capability, complexity, and dynamic performance, a fourth-order with an improved observer structure is presented to decouple the parameters between the control law gain and gain coefficients of the ESO. The proportional estimation error is introduced into the proposed DHESO to change the structure of the disturbance feedforward, eliminating the control law gain in the disturbance transfer function. In this way, the disturbance rejection capability is only influenced by the gain coefficients of the DHESO to overcome the difficulty in parameter tuning and obtain precise disturbance observation and compensation. Through an analysis of the multi-frequency characteristics, the change law of the gain coefficients with periodic disturbances at different frequencies is investigated and designed. Experimental results show that the proposed strategy has better disturbance rejection capability under different frequencies and good robustness under parameter mismatch.</p>