In hydraulic motor driving load system,speed fluctuation characteristics of motor shaft directly affect the stability and reliability of load.The dynamic model of typical hydraulic motor-load system was established,and the calculation method of equivalent spring torsional stiffness of hydraulic motor was expounded.According to the principle of nonlinear dynamics,the influence mechanism of torsional stiffness and friction torque on the speed fluctuation characteristics of the system was analyzed,and the high frequency sampling and counting method of the speed fluctuation measurement and analysis was put forward.The theoretical analysis and experimental results consistently indicate that the variation of nonlinear friction torque,output volume ripple,load torque and effective bulk elastic modulus of oil affect the speed fluctuation characteristics of the system.