Leakage is a typical fault of hydraulic cylinder,the diagnosis of leakage can effectively guarantee its efficiency and improve the degree of safety.Stribeck friction model was introduced into the model of valvecontrolled hydraulic cylinder system to establish the system dynamics equation which was close to the actual situation.Aimed at the nonlinear problem of the system,the state space model of the system was established by the state feedback method.The subspace identification method with white measurement noise was used to identify the system,and the leakage coefficient of the system was estimated to identify the existence,the type and the degree of leakage fault.The diagnosis result was verified by MATLAB/Simulink platform.The result shows that the state feedback method can establish a linear system model,so that the subspace identification method can diagnose the leakage fault.The subspace identification method with white noise can eliminate the influence of noise,and accurately identify the estimated value of each leakage coefficient.