Nowadays, more and more attention has been paid to improve the performance of the nozzle flapper servo valve. As a core part of nozzle flapper servo valve, the armature assembly is affected by electromagnetic force, jet force and feedback force at the same time. Due to the complex structure of the pilot stage flow field and the high jet pressure, the prediction of the jet force has always been difficult in modeling the transient motion of the servo valve. Whereupon, a numerical simulation method based on the flow-solid interaction(FSI) is applied to observe the variation of the jet force when the flapper is moving. Different parameters are employed to seek a suitable numerical simulation model which can balance the accuracy and computational cost. By comparing with the experiment results, the effectiveness of numerical simulation method in predicting the variation of the jet force and cavitation is verified. By this numerical simulation model, the distribution of flow field and the force on the flapper predicted by the moving and fixed flapper are compared. The results show that more dynamic details are achieved by the transient simulation. By analyzing the numerical simulation results of different inlet pressures and flapper vibration frequencies, the relationship between the movement of the flapper, the flow field distribution, the jet force and the inlet pressure is established, which provides a theoretical basis for the subsequent modeling of the armature assembly.