A numerical model to evaluate the sealing performance of the hydraulic combined dynamic seal is introduced in this paper. Based on thermal elastohydrodynamic lubrication (TEHL) theory, the impact of thermal effect on dynamic sealing oil film is emphatically considered in this model besides pressure and linear velocity. The relations between the flow field parameters and the sealing performance are established under a threedimensional model, then a numerical solution is reached and the result is analyzed.