With the continuous improvement of computer numerical control (CNC) system processing speed and machining accuracy, traditional low step mathematic model can not express the high speed movement character any more. Therefore, an integrate model for servo system is established and simplified with reason, upon which a 4th step model which embodies high speed movement character of servo system is reasoned out. By the emulation analysis of numerical control system which uses normal PID controller, the phenomenon that responds curves deteriorated with the improvement of servo system speed when servo system controller works with the same PID parameter is detected. When primary PID theory is not suitable to high speed servo system, the 4th step model should be solved in order to obtain the best parameters in terms of high speed motion. A pole planning method is proposed to optimize important parameters for high speed 4th step model of servo system, because it is very difficult to get the accurate solutions of high step equation. Sequentially, the high speed machining servo system parameters which make respond characteristics without oscillation or overshoot are obtained. Simulation proves that this method is useful and successful.