After a single phase-to-ground fault occurs in a distribution network, the asymmetrical degree of three-phase-to-ground parameters between fault line and non-fault line is significantly different. Based on this characteristic difference, an admittance asymmetry parameter is defined and a fault line selection scheme based on it is proposed. By combining the principle of the fifth harmonic, the scheme solves the problem such that the fault difference between fault line and non-fault line disappears in resonant grounding system. This makes the criterion suitable for different types of system. The theoretical analysis and PSCAD simulation result show that the method does not need to set the action threshold and has high sensitivity. The admittance asymmetry parameter also has strong ability to resist transition resistance. It can be used as an effective supplement to the existing line selection criteria. This work is supported by National Natural Science Foundation of China (No. 51877084) and Fundamental Research Funds for the Central Universities (No. 2016MS87).