The flow field inside the spool valve during spool moving was simulated and analyzed using the CFD software Fluent. The simulation results show that the transient flow force is large when the valve is in small throttle with large flow and high moving speed, which must be considered during the valve design. By the analysis of the hydraulic force act on the valve-ole, there is an additional radial force that causes valve element lockup existed. This force is difficult to be balanced with only structure method. By computing the discharge coefficient valve in static and dynamic condition, the results show that the discharge coefficient changes greatly with the spool stroke and moving state.