In order to meet the requirements of active optics control technology for segmented mirror telescope, an improved active disturbance rejection controller (ADRC) was designed to improve the position tracking performance of the position actuator and the capability of anti-disturbance. Firstly, the mathematical model of the position actuator, and the model of wind disturbance were established. The improved ADRC was described and the method of controller parameter selection was given. Secondly, the feasibility of the controller was demonstrated through a simulation of the control system of the position actuator. Finally, with the wind disturbance mechanism, the wind force was introduced into the position actuator system, and the performance of the improved ADRC, ADRC and the PID controller was compared by experiments. Experimental results show that the stability time of the step tracking of the improved ADRC is 201 ms, the steady-state mean square error is 7.1 nm, and there is no overshoot. In the anti-disturbance experiment, the maximum deviation of the improved ADRC is 38.8 nm and the steady-state mean square is 7.6 nm. These results illustrate that the performance of the improved ADRC is significantly better than those of ADRC and the PID controller, and the improved ADRC improves the performance of the position actuator system.