The model of the reciprocating motion gap seals of impact cylinder was established by using FLUENT software,and the gap sealing performance was studied by combining experimental test system.The effect of piston velocity and pressure difference in the inlet and outlet of sealing clearance on the leakage of the gap sealing was studied to control and reduce leakage of reciprocating motion gap seals.The results show that when the sealing gap of impact cylinder is not changed,the leakage rate is increased linearly with inlet pressure increases,and the change rate of the pressure difference and the amount of leakage are not affected by the piston speed.The leakage rate is increased linearly as the piston speed increases at the same sealing gap and inlet pressure.The operating pressure has little effect on the amount of leakage due to its small change range,but it can not be ignored the effect of pressure changes on the impact velocity,which leads to increasing of the amount of leakage.