When single-phase grounding fault occurs in resonant grounded system in different fault conditions, the fault signals are very weak, which makes it difficult to detect the fault. According to the differences in characteristics of transient zero sequence current of short-time window time-frequency, the transient characteristics of the zero sequence current of single-phase earth fault is utilized and analyzed in this paper by S transform, where the zero-sequence current amplitude time-frequency matrix can be constructed based on these differences in characteristics. On the basis of multiple frequency information and local characteristics of time-frequency, a new method of fault line selection based on S-transform time-frequency characteristics is proposed in this paper. This method can be applied in the scenarios with the lack of information coverage in a single feature band and the inconsistent band characteristics of each feeder. Combined with the similarity assessment of image processing, the single-phase ground fault line selection for a resonant grounding system of a distribution network can be achieved by comprehensive correlation coefficients. The simulation results show that the new method will be less affected by fault initial angle, fault resistance, fault location and network structure. It can effectively distinguish different fault grounding types, and has obvious fault boundary, high reliability, strong anti-interference ability, and more extensive applicability.