Due to the disadvantages which the PI control have such as relatively large overshoot, slow dynamic response and other shortcomings in the three-phase four-wire active filter DC-side voltage, design a mixed fuzzy PI controller combined fuzzy control with PI control. The controller fuzzes input variables and produces output variables which are fuzzy controlled by fuzzy inference, in order to achieve the purpose of modifying the parameters online. Analyze the basic principle of DC voltage PI control, use the dual-output of fuzzy controller to adjust the parameter of PI controllers and give a two-dimensional fuzzy PI controller design process. MATLAB simulation results show that this method produces small overshoot, fast dynamic response. Therefore, this method meets the requirements of mutation load conditions and finally verifies the validity and superiority of the fuzzy PI controller.