In this study, based on the variable speed scanning of the permanent magnet synchronous motor (PMSM) in the high precision servo system, we designed an improved auto disturbance rejection controller (ADRC). The aim of our design is to address the disadvantages of the complex parameters and the delay response of the trace differentiator (TD) and expansion state observer (ESO) module. The improvement simplified the TD and ESO module, and the adjustable parameters were minimized. This helps the motor to adapt to an environment with large internal and external interferences, and to achieve high precision and a rapid response. The improved ADRC was then employed in the servo pendulum scanning mechanism of an actual project, and its performance was compared to a traditional PID controller under similar conditions. The results confirm that the improved ADRC performs much better than the PID as follows: the 0°/s.10°/s response time is 75 ms, the overshoot is less than 6%, while the steady-state accuracy is within ±1%. Further, the speed fluctuation of the variable speed tracking process is small without overshoot, the scanning cycle time fluctuation is less than 0.0041 s, and the starting angle positioning accuracy is better than 0.0015°. The improved ADRC corresponded with the actual project index requirements and has certain reference value for other systems with PMSM to achieve variable speed tracking scanning.