The main executive element of the retractable landing gear is the hydraulic cylinder, and its common fault is caused by the leakage of the hydraulic cylinder. In this study, the mathematical model of the energy entropy of the wavelet packet is constructed according to the fault characteristic information, and the hydraulic system is modelled by the Amesim software. Through the simulation calculation of the leakage and leakage of the hydraulic cylinder, the fault characteristic parameters such as displacement and pressure are extracted. Wavelet packet transform is used to obtain the energy entropy of the wavelet packet, and a mapping relationship between the leakage of the hydraulic cylinder and the energy entropy is established. The research shows that the leakage fault diagnosis method based on wavelet packet can effectively predict the failure of the hydraulic cylinder of the landing gear, and it is effective and practical for guiding the fault prediction and intelligent maintenance of the hydraulic system of the aircraft.