The single-phase grounding fault has the highest probability of occurrence in the grounding system of the arc suppression coil at the neutral point, and the electrical fault characteristics of the grounding point are weak, so the line selection is difficult. The amplitude and phase characteristics of the 5th harmonic component of the capacitance current mutation of different lines are obviously different. By analyzing and comparing these differences between the non-faulty line and the faulty line, the line selection method combing the 5th harmonic component based on the mutation of the capacitance current and Duffing oscillator detection principle is proposed in this paper. The capacitance current mutation of each phase and each line is added as an external driving force to the Duffing oscillator subsystem, and the consistency of the phase trajectory state changes of each phase of the line is observed. The fault line can be selected, and the Matlab simulation results show that the algorithm has higher accuracy.