Founded a certain ground-to-ground missile’s motion model, designed the missile’s lateral accelerate autopilot in pole placement with states constraint, analyzed guidance loop’s characteristic, and the feasibility of applying GPS achieve proportional guidance. Analyzed the impact of refreshing frequency of guidance command and the GPS ’s error to the guidance system’s accelerate and the miss distance, the results show that increasing the terminal guidance time will reduce the effect of GPS’s refreshing frequency and location error. Choose the refreshing frequency for a certain missile’s guidance loop.