Abstract: The paper proposed a diagnosis method of single-phase grounding fault of distribution network based on fuzzy reasoning, namely adopting fuzzy theory to improve three traditional diagnosis methods of single-phase grounding fault including amplitude comparison method of zero-sequence current, method of summation of mutual integral of transient zero-sequence current and signal injection method, and fusing diagnosis results of the three methods for comprehensive judgement by use of fuzzy reasoning theory. For problems of numerous, complicated and short branches of distribution network, the paper proposed a network layering method and used the diagnosis method to judge fault of layered distribution network layer by layer for a final diagnosis result. The analysis result showed that the fault diagnosis method and network layering method can judge fault correctly and quickly.