In view of the vibration phenomenon of servo systems with large load at low speed,the flow nonlinear characteristics of the servo valve with a small opening were discussed.Based on the transition assumption of flow regimes change,a piecewise linear flow coefficient model was proposed to describe the change law of the flow coefficient with the flow regime and the Reynolds number during the gradual opening of the servo valve throttle.The experimental results show that the flow coefficient model can be used to accurately describe the flow characteristics of the servo valve with a small opening,which provides a theoretical explanation for the flow gain fluctuation,and it is preliminarily determined that the critical Reynolds number for the transition from laminar flow to turbulent flow is 55.The research results establish a foundation for more accurate nonlinear modeling and improving the control quality of valve control systems.