The return voltage method is an effective and nondestructive diagnostic method in time domain. In view of the polarization spectrum of the return voltage implying rich aging information of the transformer oil-paper insulation, this paper proposes an analysis method for the return voltage polarization spectrum based on wavelet packet transform. The 3 layer’s decomposition of wavelet packet is exploited to obtain the each frequency band of return voltage polarization spectrum, and also the energy feature vector of return voltage polarization spectrum is constructed for analyzing the aging condition of transformer oil-paper insulation. In order to verify the reliability of the method, it uses the energy feature vectors which are abtained by 3 layer’s decomposition of wavelet packet of return voltage polarization spectrum of 2 different aging state’s transformer to analyze the aging of insulating medium. The analysis results show that the energy feature vector of return voltage polarization spectrum derived from the wavelet packet decomposition method can effectively diagnose the aging condition of transformer oil-paper insulation, which provides a new method for aging diagnosis of transformer insulation.