An active disturbance rejection controller (ADRC) applied in high-precision tracking system is introduced. Comparing with conventional proportional-integral-differential (PID) controller,the function analysis of the controller is performed. With an actual system,control advantages of the controller are analyzed. PID and ADRC controllers are used respectively to design the speed loop of the high-precision tracking system. Consistent results from MATLAB simulation and experimental verifications show that the speed response characteristics of the servo system are improved by ADRC controllers,which can realize fast step response without overshoot. ADRC also has better tracking accuracy in terms of stability and robustness than that of conventional PID controllers in sudden disturbance.