In order to study the dynamic leakage characteristics of the plunger pair of high pressure common rail injector, a mathematical model of the fuel flow in the gap between the plunger pair was established. The finite difference method was used for numerical calculation of the mathematical model, and the changing trend of the average dynamic leakage rate of the plunger pair fuel under different rail pressure was studied. The comparison with the test value was made to verify the validity of the mathematical model. The effects of rail pressure, inlet fuel temperature, plunger maximum lift and injection pulse width on the dynamic leakage of the plunger pair were analyzed, and the changing trend of oil film pressure, thickness and temperature parameters at different plunger speeds were also analyzed. The results show that the change curve of the dynamic leakage rate is similar to that of the control chamber oil pressure in a fuel injection cycle. The average dynamic leakage rate increases with the increase of the rail pressure of the injector, the inlet fuel temperature and the maximum lift of the plunger, but decreases with the increase of the injection pulse width. As the speed of the plunger changes from positive speed to negative speed, the thickness of the oil film becomes thinner, the temperature of the oil film rises as a whole, and the pressure of the oil film becomes smaller.