According to the characteristics of a large optical telescope by large inertia and nonlinear frictions, an Active Disturbance Rejection Controller (ADRC) was designed to improve the response speed of the servo system. The working principle and basic structure of the ADRC was introduced,the regulation of parameters for the ADRC was given, and each parameter was analyzed imitatively. Finally, the ADRC was compared with conventional PID controllers. Actual results show that the ADRC can not only achieve a higher response speed without a overshoot, and its low-speed step response time can be shortened and the low-speed smoothness be improved.When it runs in 0.005 (°)/s, the system settling time is 1 s, the standard deviation of velocity fluctuation is 0.000 082 (°)/s (maximum in 0.000 42 (°)/s).The results demonstrate that the performance of the ADRC is better than that of the traditional PID controller. Experimental results show that the ADRC can inhibit friction, saturation and other nonlinear factors, and can improve the speed performance of the telescope servo system.