Based on analyzing the working process of switch solenoid valve, the design method of PWM frequency based on the valve's critical frequency and cutoff frequency was given, by using PWM technology to control the valve. Using the switch valve's voltage balance equation, kinematics equation and fluid equation, the expressions of the critical frequency and cutoff frequency were given. The experimental results show that the critical frequency and cutoff frequency are basically the same with the measured values and the error meets engineering design standards. The relationship of the PWM duty ratio,the flow and the PWM frequency was analyzed.