There exists serious hydraulic impact in pump of wet spraying system when the valve changes its direction, and the main cause is that the kinetic energy of the oil and load is transformed into pressure energy of the oil and load is related to the displacement of the pump. A method to control the hydraulic impact by the active reduction of the displacement of the pump was proposed. Through AMESim modeling and simulation experimental study, this active control method was proved feasible. It provides a theoretical basis for optimal design of pump of wet spraying.