Abstract: After the single-phase high-resistance grounding fault occurs in the flexible grounded system, the input of the parallel low resistor will lead to the reduction of the steady-state fault component. At the same time, the variation of the steady-state amplitude of the zero-sequence current of each feeder is small, so it is difficult to select the fault line. The transient fault characteristics of the flexible grounded system after the parallel low resistor is connected are analyzed; the inherent relationship between the feeder transient zero-sequence current and the bus transient zero-sequence voltage and their derivatives is obtained; and the relationship between the time when the low resistor is connected and the transient fault characteristic amplitude is analyzed.Thus a transient fault line selection method for the flexible grounded system based on the transition resistance evaluation is proposed. This method evaluates the transition resistance through the low-frequency transient voltage and current generated after the connection of the low resistor, so as to realize the fault line selection, which is not affected by the initial phase angle of the fault, has strong resistance to the transition resistance, and has low requirements for the accuracy of the transformer. At the same time, the maximum regulation of transient fault characteristics can be realized by selecting the connection time of low resistor,which can provide a new idea for flexible grounded system line selection and protection methods. Finally, the correctness of the proposed method is verified by simulation and real time digital simulation（RTDS） experiments.