It is difficult to accurately identify the fault phase and cause of wildfires due to a tree-line fault of three-phase voltage imbalance in a distribution network. Thus a phase selection method based on the combination of voltage signal mutation value and change trend is proposed. Based on the phase selection theory of a single-phase grounding fault in a traditional three-phase voltage balanced distribution network, a 10 kV single-phase tree-line fault test platform is built. First, a single-phase tree-line fault test of various trees and different residual currents is carried out under three conditions of neutral point ungrounded, under compensation and over compensation of arc suppression coil grounding. Then, from the variation characteristics of zero sequence voltage and phase voltage amplitude during a tree-line fault, the fault time and fault phase identification code are defined, and the criterion that the lag phase is the fault phase is established when the amplitude of the effective value of a phase voltage increases the most. By comparing and judging the change of the average effective value of each phase voltage within 0.1 s before and after the fault time, the corresponding identification code is generated for matching to achieve accurate and rapid identification of the fault phase. Finally, it is verified by experiment that this method can effectively solve the problem of fault phase identification of a single-phase tree-line fault and is not affected by voltage imbalance. This has good engineering application value.