Positioning accuracy of servo feed unit makes an important effect to processing precision in CNC machine tools. The effects in positioning accuracy mainly are the precisions of mechanism and servo system. And the actions of tuning controller gains should directly affect the servo system precision. This paper, a servo system model and its P/PI controller has been formulated and simulated, using an AC servo feed unit as a tested. Through tuning gains of P/PI controller, analyses and control the effects for the feed unit positioning accuracy. Ultimately, experiments have been carried out to test feed unit by laser interferometer. Its shows that tuning gains of P/PI controller, which given in paper, has effectively improved the positioning accuracy of servo feed unit.