Zero sequence overcurrent protection adopted in the flexible grounding system faces the problem of low tolerance to transition resistance. The existing fault line selection methods also require high-precision current transformers and do not take into account the influence of grid parameter imbalances, increasing the risk of protection misjudgment. To address the issues, this paper first explores the relationship between the changes in electrical signals and line-to-ground parameters and unbalanced current before and after the introduction of a small parallel resistance, taking into account the unbalanced conditions of grid parameters. The research finds that the ratio of zero sequence current change and zero sequence voltage change of a healthy line is equal to the line-to-ground admittance, and the ratio of the change of the fault line is the sum of line-to-ground admittance and grounding admittance. On this basis, a method of line selection applicable to single-phase high-resistance ground faults in flexible grounding systems is proposed. This method is not affected by the unbalance of grid parameters and the zero sequence CT polarity reversal, effectively reducing the precision requirements of current transformers. In addition, the method can also utilize the fault information to measure transition resistance and the ground admittance of individual lines under unbalanced grid parameters. Finally, the correctness and effectiveness of the described method are verified in the simulation circuit model.