The injection pressure and volume efficiency of common rail highpressure pump are limited by the piston pairs interface leakage,which is influenced by the micro motion characteristics of piston.Based on the structural dynamics and the hydrodynamic lubrication theory,the piston pairs’ kinetic model was established,and the micro motion characteristics,such as the offset and the inclined angle of piston,were analyzed.By considering the effects of the piston micro motion,the elastic deformations of piston pairs and the change of the physical properties of fuel comprehensively,the piston pairs’ transient leakage model in the ascending section was established by comprehensively considering the effect of the piston micro motion,the elastic deformations of piston pairs,and the fuel property changes,and the characteristics of the piston pairs interface transient leakage was studied.The calculated result was validated with the measurements.The calculation results indicate that in the ascending section,the offset of the piston is first increased and then decreased,the inclined angle is first decreased and then increased.Compared with the transient leakage measured,the average error of the leakage calculated based on the model considering the micro motion of piston is only 3%,however,the average error of the leakage calculated based on the model without considering the micro motion of piston is almost 20%.Therefore,the model considering the micro motion of piston is more precise.