Abstract: In view of the issues that the control effect of the load system of friction welding machine on pressure mutation phases is unsatisfactory with(proportion integration) closed-loop control algorithm, the fuzzy PI control algorithm is introduced. According to the error and its changing trend of the actual value and the set value, the whole control process is divided into four kinds of control intervals. Based on optimizing the fuzzy control degree, the corresponding fuzzy rules and parameters determining methods are developed. Under the test conditions, the control effects of four control intervals with the different control parameters are contrasted, the best fit coefficients of every control interval are obtained. The performance indicators using fuzzy PI control methods are better than that using traditional PI control methods. Under test conditions with the optimizing fuzzy control degree and the best fit coefficients, the maximum overshoot has been reduced 19.2%, the adjust time has been reduced 1.59 s, and the steady precision has been raised 0.4%.