Abstract: For problem of difficult line selection caused by small current of fault line with uncertain direction when single-phase grounding fault happens in resonant grounding system, the paper theoretically analyzed change law of zero-sequence voltage and zero-sequence current at the time of single-phase grounding fault. It built simulation model of small current grounding system in Matlab and did simulation analysis of isolated neutral system and resonant grounding systems under conditions of full compensation, over compensation and under compensation separately. The simulation analysis result was the same as the theoretical one. Finally it got conclusions, namely when single-phase grounding fault happens, isolated neutral system can use line selection methods based on steady state component of amplitude and direction while resonant grounding system cannot, and the both systems can use the same line selection method based on transient state component.