The invention discloses a single-phase earth fault line selection method of an undercurrent earthing system. The single-phase earth fault detection method is high in detection sensitivity and does not need installing signal injection equipment, and includes the steps: starting through zero sequence voltage, by means of the zero sequence current data after and before occurrence of the single-phase earth fault, after band-pass filtering, calculating the current vector of each phase and the vector product of the corresponding variance vector; and according to whether the vector product is zero, determining the earth phase, and then performing earthing line selection by comparing the magnitude of the current variance of each outgoing line of a transformer station. The single-phase earth fault line selection method of an undercurrent earthing system can escape from the influence of the transient process and the initial fault current angle without being influenced by the neutral point operation mode, does not need installing the signal injection equipment, and is suitable for an overhead line, a cable line or a hybrid line.