The mechanism and producing condition of instsbility in low velocity of hydraulic clinder was analyzed based on dynamic model and test ,Nonlinear coupled force combined with hydraulic spring and Self-excited vibration were though the key factor that could effect instability in low angular velocity, The mathematic model was established to obtain the change rule of speed & instantaneous acceleration of the hydraulic clinder A simulation analysisis then created for dynamic characteristics of hydraulic clinder with Simulink to analyze the effect of friction torque, leakage and compressibility of oil and liquid on low-speed stability of hydraulic motor, The test shows that fluctuation was the negative characteristics of friction in the self-excited vibration conditions .Some measures were proposed to improve the low-speed stability of hydraulic clinder.