Abstract:The cylinder is the executing agency of hydraulic system, the main performance of the motion fault of which is power shortage, which will cause its action is not timely and soon. According to the complex characteristics of fault appeared in the hydraulic control system, the hydraulic cylinder leakage is researched as an object. The analysis was carried out of the fault form in leakage degree of hydraulic cylinder, and then the leakage degree was classfied, the leakage fault diagnosis method of hydraulic clinyder was proposed based on LS-SVM, and fault diagnosis model of the system was established. Quantative analysis and alogrithm research were carried out on the leakage rate of cylinder. Researches show that this method of the hydraulic cylinder leakage fault diagnosis aimed at principle, method and characteristics can be more accurate to predict the degree of hydraulic cylinder leakage, its fault diagnosis is effective and practical.