For the hydraulic seal, the pressure of the fluid film in the seal area is not equal to the seal pressure which leads to errors in the study of oil leakage, and insufficient considerations in the study of the seal ring seal performance, the fluid film pressure was studied from the sealing mechanism of the sealing ring. A fluid film model of the seal ring under oil pressure was established based on the Laminar model in Fluent. Finite element analyses of the seal ring in static seal and dynamic seal were carried out to obtain the accurate pressure distribution in the seal area. The theoretical calculation of the leakage amount and the finite element numerical calculation after the fluid film pressure was used to instead of the sealing pressure were compared and analyzed. The research results show that the leakage amount calculated by using fluid film pressure instead of the sealing pressure is more accurate and the error is smaller; the fluid film pressure is affected by the sealing pressure and the piston rod speed; the leakage is directly proportional to the sealing pressure and the speed of the piston rod, and the speed of the piston rod has a greater impact on the fluid film pressure and the leakage. Obtaining the precise pressure of the fluid film provides reference for the study of the sealing performance of the seal ring, and lays a certain foundation for obtaining its accurate sealing mechanism.