To improve the accuracy and response performance of servo system, put forward a nonlinear PID control scheme. After comparing the performance of acquiring and tracking response, the design method of the controller which fast response and stable is described. Then the method is verified and analyzed in an electro-optical tracking system. The result shows that nonlinear PID control scheme not only increases the speed of closed-loop response, but also improves the settling performance while switching acquisition to tracking modes, and successfully used in the application.