Abstract: In order to improve the machining precision of ultra precision machine tools, a kind of double-mode and parameter-adjustable fuzzy controller is designed. It can adjust the parameters and alternate the fuzzy control by itself according to the errors and error changes. In reference to the AC servo system of ultra precision machine tools, simulation analysis is performed for the fuzzy controller and PID controller. It shows the tracking characteristic a position precision of the fuzzy controller better than that of the PID controller. The result shows that the fuzzy control system is well adapted to the non-linear characteristics of the AC servo system that is better than PID controller.