It is difficult to select a fault line when a high impedance grounding fault occurs in a flexible grounding system. Thus a fault line detection method for such a system based on zero-sequence current amplitude ratio multiplication coefficient is proposed. First, the variation characteristics of amplitude ratio of zero-sequence current at neutral point to zero-sequence current of line before and after a parallel small resistor is put into use in the single-phase grounding fault of the system are analyzed. This means that the amplitude ratio of zero-sequence current at the neutral point and the zero-sequence current at sound line increases, while the amplitude ratio of zero-sequence current at neutral point and zero-sequence current at fault line decreases. Second, by introducing the multiplication coefficient, the changing trend of amplitude ratio is quantitatively described, and then the adaptive fault line detection criterion is constructed. In addition, the neutral point inductance and resistance are used to indirectly determine the multiplication coefficient of the sound line with weak zero-sequence current amplitude after the input of a small resistance. This effectively reduces the misjudgment probability of line selection caused by the limitation of zero-sequence CT accuracy. The theoretical analysis and simulation results show that the proposed method can withstand a transition resistance up to 3000 Ω without zero-sequence voltage information, and is reliable and applicable. This work is supported by the National Natural Science Foundation of China (No. 52207075 and No. 52037001).