Abstract: To overcome the effects of the parameter variations, cogging effects, load disturbance and degrading redundancy and achieve high-performance servo control, an active-disturbance rejection controller (ADRC) of multiredundancy brushless DC motor servo system was presented. The position controller was composed of tracking differentiator (TD), extended state observer (ESO) and nonlinear state error feedback control(NLSEF) law. The TD provided a transition process for reference position, which overcome the contradiction between responding speedily and overshooting smally. The ESO observed and compensated the disturbance caused by parameter variations, cogging effects, load disturbance and degrading redundancy. The NLSEF implemented a nonlinear control and led to accurate results. Simulation results demonstrate that this controller has good dynamic and static performances and strong robustness against disturbances.