In closed piston pump, the “positive opening” flow distribution structure is adopted to ensure the small flow distribution impact under all working conditions. During the flow distribution process, when the pressure difference between the inlet and outlet of the damping groove changes, the pressure difference flow with a higher speed is generated in the groove, which leads to low pressure area and serious distribution cavitation. In order to ensure the smooth change of oil pressure in the flow distribution process of the closed piston pump and reduce the flow distribution impact and noise, the cavitation distribution and gas volume fraction under all working conditions were studied by using PumpLinx software. The results show that with the increase of displacement, working pressure, speed, oil temperature and the decrease of oil bulk elastic modulus, the gas volume fraction in the flow field area of the valve plate increases and the cavitation phenomenon intensifies.