Because Electromechanical Actuator (EMA) is a nonlinear, time-varying servo system, this paper proposes an improved Active Disturbance Rejection Controller(ADRC) to improve the tracking performance of the EMA. First, the mathematical model and control strategy of the EMA were presented and the nonlinear factors influencing the performance of EMA were analyzed by the control theory. Then, the improved ADRC was described and the method to select parameters were given by modern control theory. Furthermore, the feasibility of this controller was demonstrated through a simulation under different input conditions. Finally, the performance of PI controller, ADRC, and improved ADRC was compared by experiments on a harmonic drive EMA servo system. Experimental results indicate that the improved ADRC controller can remove the position flat crest and velocity dead space, and its phase error is 0.087 22 rad when the angular position signal of 10 sin(5 πt) is tracked. Furthermore, the rise time, overshoot and the steady state mean square deviation of EMA system are 9-18 ms, 0-7.25%, 0.007 60-0.010 83, respectively, when ±1°-±15°angular positions are tracked. These results means that the performance of the improved ADRC is better than those of the PI controller and traditional ADRC. It has a fast response, slight overshoot and high accuracy in stability, as well as strong anti-disturbance and robustness.