Being as the key component in hydraulic control systems, hydraulic servo-valve plays an important role in the systems. The characteristics of a hydraulic servo-valve may influence the performance of a hydraulic control system significantly. The characteristics of a servo-valve will be deteriorated or even failure of the valve will be caused when high frequency self-excited noise and oscillations appear inside a servo-valve. Due to the coupling and interaction among the electromagnetic field, mechanical and flow field, as well as the other physical fields inside the servo-valve, it is reasonable to predict that the fluid-structure coupling is one of the reason for the appearance of self-excited noise in a servo-valve. Therefore, a method based on the fluid-structure coupling is presented in this thesis to study the mechanism of the self-excited noise inside a servo-valve. The research is carried in order to obtain reliable design and operations of hydraulic servo-valves and systems.　　A method to study the self-excited noise is developed by using fluid-structure coupling. The possibility of fluid-structure coupling between the pressure pulsations of flow field and the vibration of armature assembly is investigated.　　In this thesis, interests are focused on mechanism study to enhance possibility and understanding of the fluid-structure coupling between the flow field and armature assembly of torque motor in the pilot stage of hydraulic servo-valves by simulation and experiment. The results of the study present the understanding of the fluid-structure coupling in the pilot stage of hydraulic servo-valves due to its important consideration during design and operation of the hydraulic control system.　　Therefore, characteristics details of coupling and interaction in the pilot stage significantly influence the performance of the hydraulic control system and these are industrial needs for better models. In this thesis, mathematical models for the vibration characteristics of armature assembly are built based on the structure mechanics. And the mathematical models of jet flow in servo valves are established based on the current state of the jet flow field research and fluid mechanics theory to support simulation and experiment to the proposed method. A mathematical description of the forces on the armature assembly due to the jet flow from the nozzle and the fluid inside the chamber between the main spool and the nozzle is built for the study of coupling from the fluid domain to the armature assembly.　　The vibration characteristics of armature assembly are studied by means of simulation and experiments. The finite element models of armature assembly under different installment ways are built based on ANSYS software. The natural vibration frequencies and corresponding vibration modes are calculated through modal analysis. Through analysis the vibration mode shape at different order and different frequency are obtained during simulation which was carried during feedback rod with and without restraint. The effects of the load and the damping on the vibration characteristics are presented. To compare with the results of the simulation, the experiments of static and dynamic response are performed by using laser displacement sensor, although, the effectiveness of the finite element modal is validated by error analysis. Also by using ANSYS soft ware the harmonic response analysis of the armature assembly with the pressure pulsation of flow fields is carried out based on the modal analysis method. The harmonic curves are obtained and regularity of the vibration energy distribution is analyzed.　　The flow fields have been presented in were the 2D and 3D flow field model near the flapper nozzle is established and meshed by CAD and GAMBIT software, and simulated by FLUENT software. The finite element analysis of jet flow field with different shapes, different boundary conditions, and different zone of wake are carried out. Also the corresponding distributions of pressure and velocity are simulated, and then the condition and law of the high frequency pressure oscillation are summarized. Visualization of the jet flow field is carried out by a high speed video camera with different inlet pressure and at different shooting frame number. The jet flow oscillation period and frequency are estimated by observing the edge motion of the jet flow.　　Finally, the natural frequency of vibration of armature assembly and the frequency of flow field in the pilot stage are compared to investigate the possibility and mechanism of fluid-structure coupling in a hydraulic servo-valve.　　This study has provides some theory basis to understand and awareness of vibration characteristics and jet flow field characteristics in the fluid-structure coupling between the flow field and torque-motor armature assembly in the pilot stage of hydraulic servo-valves. In which the results offer useful reference for the discovering of the mechanism of self-excited noise and seeking of some effective restraining measures, also provide guidance to, mechanism and controller designers and asses the value of coupling at the initial design stage of the pilot stage of hydraulic servo-valve