A finite element model for “O-ring + rectangular slip ring” composite seal commonly used in aviation hydraulic actuators, was established by using the software ANSYS Workbench. The leakage and friction of the sealing ring under different squeeze, temperature and oil pressure were studied. The results show that the leakage of the seal is reduced but the friction is increased with the increase of the squeeze or the increase of the temperature. With the increase of the oil pressure, the leakage and friction of the seal are increased. Under different squeeze, temperature and oil pressure conditions, the friction of the piston rod when it is outstretched is always greater than that at the time of retraction. By changing the structure of the rectangular sliding block, the sealing performance of the combination seal was optimized. The results show that the leakage can be significantly reduced by selecting the oil side contact angle and the air side contact angle properly.