The single-phase ground 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, and the line selection is difficult. The amplitude and phase characteristics of the 5th harmonic component of the capacitance current sudden change of different lines are obviously different. By analyzing and comparing these differences between the non-faulty line and the faulty line, the 5th harmonic based on the sudden change of the capacitance current is proposed. The line selection method combining the component and Duffing oscillator detection principle, the capacitance current sudden change of each phase of each line is added as an external driving force to the Duffing vibration 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.