Aimed at the fact that the piston of axial piston pump exists processing taper and inclination during operation, the leakage flow model of piston pair was built under the premise of comprehensively considering the influence of these two factors. Based on the leakage flow model, the influence of the critical radius gap of piston pair in oil expulsion area and the two factors on leakage flow were obtained by analyzing the leakage of piston pair. The results show that the two factors have significant influence on the leakage flow; the critical radius gap can provide reference for the design of piston pair and the determination of the flow leakage direction in oil expulsion area.