The finite element model was established by ABAQUS software for the O-ring in hydraulic valve system with the piston velocity of 4 m/s，medium pressure of 6 MPa，friction coefficient of 0.3. The effect on dynamic sealing property of O-ring in hydraulic valve system by different motion velocity，medium pressure，pre-compression rate and friction coefficient were analyzed. The results indicate that the contact pressure in sealing surface of O-ring fluctuates with the displacement changes. The average contact pressure is increased linearly with the increase of work pressure and pre-compression rate, and motion velocity has little influence on the average contact pressure. The fluctuation of contact pressure curve is increased with the increase of work pressure, pre-compression rate and motion velocity. The dynamic sealing performance of contact surface between O-ring and cylinder is better than that of contact surface between O-ring and piston. The dynamic sealing performance of O-ring in push travel is better than that in return travel. When the pre-compression rate is less than 10%, the dynamic sealing performance of O-ring can not meet dynamic sealing requirements of this hydraulic valve system.To ensure the dynamic sealing of O-ring, it needs to choose the reasonable pre-compression rate.