Abstract: Selecting a reliable characteristic quantity is the key to improve the detection accuracy of single-phase grounding fault in neutral indirectly grounding system. On the basis of analyzing the similarity of waveform between zero sequence voltage and the change of system capacitance charge at the time of a singlephase grounding fault occurs within the boundary, as well as the similarity of waveform between zero sequence voltage and the change of system capacitance charge at the time of single-phase grounding fault occurring outside the boundary, a method suitable for the detection of small current grounding fault is proposed, which uses the realtime value of zero sequence current at the time of a single-phase grounding fault to calculate the real-time value of the system capacitance charge changes at the time of a single-phase grounding fault, and to calculate the similarity coefficient of waveform between zero sequence voltage and the system capacitance charge changes at the time.Based on the obvious discreteness of the similarity coefficient, the inside and outside attributes of a single-phase grounding fault are judged.