In order to quantitatively analyze the internal leakage of high-precision electrohydraulic servomotor, a numerical method to solve the oil-film control equation is proposed based on the finite element method. In this method, the contact stress on the seal surface is calculated with the finite element software ADINA; the oil film thickness is obtained by solving the one-dimension Reynolds equation with the inverse method; and the leakage and the friction are also calculated. Then, in order to overcome the difficulty in determining the inflection point of the two-order derivative pressure function, a cubic polynomial is used to fit the pressure at the inlet. Moreover, the effects of O-ring pre-compression ratio, medium pressure, rotor radius, motor velocity and temperature on the leakage and the friction are analyzed. The results indicate that the proposed numerical method is more stable and effective than the conventional iterative method. It is applicable for the calculation of leakage and friction of electrohydraulic servomotor, and it lays a theoretical foundation for the design of elastomeric seals.