Through the analyzing of the leakage of each channel in the double-acting pump with composite vanes, the mathematical model of internal leakage is established. According to the mathematical model of leakage, the leakage size of each passage was calculated. It was found that axial clearance leakage was the main internal leakage way, and the distribution graph of leakage with varying pressure and viscosity was plotted. When the viscosity was low, the amount of leakage would be increased sharply with the increase of pressure. The influence of temperature and rotational speed on the volume efficiency of leakage loss was analyzed. It is concluded that the volumetric efficiency loss will increase exponentially with the increase of axial clearance. In order to reduce the influence of leakage on volumetric efficiency, the axial clearance must be strictly controlled in the design, manufacture and installation of the flow distribution plate.