In the PID controller design, three sensitivity functions are used as the design indexes of mixed sensitivity. In order to select the weighted functions of the three sensitivity indexes, a method of determining the weighted functions based on dynamic tracking performance is proposed. The method is applied to the performance test of the servo control system of steering gear. The test results show that the hybrid sensitivity is used as the design index of the servo system PID control, and the control system has strong anti-interference ability and good tracking performance.