Abstract: It starts with the analysis of single-phase leakage fault of neutral ungrounded system, and uses the Venin theorem for steady-state analysis, and obtains the relationship between zero sequence voltage and zero sequence current of fault line and non-fault line when fault occurs. Based on the principle of zero sequence power direction,the relationship between the branches when the fault occurs is calculated and the line selection criterion is finally obtained. In order to verify the correctness of the theoretical analysis results, the analysis in extreme cases was carried out in the end to verify the correctness of the equivalent circuit and the theoretical analysis, as well as the accuracy and universality of the fault line selection criteria.