Abstract: In view of frequent single-phase grounding fault in distribution network system,because the fault point current is very small, so judging fault line accurately becomes very difficult through fifth harmonic amplitude comparison . The method of fifth harmonics and the combination of wavelet packet energy are presented. Use wavelet packet to do three layer wavelet packet decomposition on five harmonic of zero sequence current of each line , then calculate the total energy on the third floor reconstruction coefficient. By comparing the fifth harmonic energy of the zero sequence currents ,the fault line can be judged.By setting up 10 kV neutrall point via arc suppression coil grounding current of single phase short circuit grounding system simulation model, the results show that the method of fifth harmonics and combination of wavelet packet energy can accurately select fault line .