The power control valve of the hydraulic plunger pump can automatically reduce the pump displacement with the increase of the hydraulic system pressure when the pump reaches the starting point of power at constant input speed,and can insure that the output power of the pump is basically constant.The power valve in the hydraulic system of sand throwing fire extinguishing truck can meet the working condition requirements of the sand throwing fire extinguishing truck,reduce the pump displacement and increase the output torque under the condition of blocking or starting with load.The system can discharge and spray the material with a larger pump displacement under normal conditions,so as to improve the distance and quantity of the material.