The fluid flow of piston cavity is the basis for researching the pressure of engagement process of wet clutch. Through simplifying geometric model of the piston, by using the k-εmodel in FLUENT and the method for solving non steady sliding mesh, the rate of flow and change of pressure were calculated by making numerical simulation about fluid distribution in the piston cavity during engagement process of wet clutch. The results show that FLUENT can simulate the flow field movement greatly about the fluid relative balance which in the airtight and empty rotating container with constant angular velocity. The simulation calculating results and theory analysis are in good agreement with each other. The calculation has a reference value for the research about wet clutch piston pressure distribution law and the oil-pressure calculation of engagement of wet clutch.