In order to analyze the power torque characteristics of wet clutch produced under the disengaging state and reduce the drag torque of wet clutch, a flow rate equation under full film state was deduced based on the Navier-Stokes equations and a new equation calculated oil film equivalent radius was derived according to the relationship of oil flow rate and volume between import and export, whereafter the drag torque model considering the friction gap non-uniform was established based on the Newton internal friction theorem with front formula. The performance of simulation shows that the drag torque increased monotonically with the rise of speed in low speed. But when the speed increased to a certain value, the drag torque began to decrease. Then, two evaluation indexes for drag torque were presented which were peak value of drag torque and the corresponding critical speed. Subsequently, the sensitivity for the effect of clutch parameters on drag torque was calculated. The conclusion of research have some reference value to the design of wet clutch.