The feature extraction of Hbridge inverter electrolytic capacitor parameters fault diagnosis is researched. The capacitor aging characteristics is analyzed by the traditional fundamental equivalent analysis based on the fundamental harmonic. In the actual situation, this method has large error and its reliability is poor. In order to solve the above problem, this paper puts forward to use the wavelet packet frequency band energy method to extract the fault feature. This method can accurately extract the signal band energy changed information. Through building the simulation model of Hbridge inverter to simulate the different capacitance aging fault, the band energy can be extracted under the capacitor normal and varying degrees of aging based on the wavelet packet frequency band energy method, feature vectors are constructed, and Hbridge inverter electrolytic capacitor characteristic change rules are sought as the capacitor aging. Simulation results show that this method can accurately extract the Hbridge inverter electrolytic capacitor aging fault feature.