A tree-to-wire fault (TF) in medium voltage distribution lines can cause wildfires and serious safety accidents. Sensitive and accurate TF identification is of great significance in ensuring power supply and public safety. Based on the publicly available experimental data of the “Vegetation Conduction Ignition Tests” in Australia, for tree-contact single-phase-to-ground fault (TSF), an analysis of the entire physical process from contact to fire ignition reveals that over 95% of TSF exhibit a similar impedance gradient pattern in the initial stage. The tree impedance shows a double exponential decay characteristic due to discharge heating. Also a method for detecting and identifying impedance gradient properties based on double exponential curve fitting is proposed. The proposed method is validated using PSCAD/EMTDC simulation data, public experimental datasets, and real experimental test data. The results show that the proposed method can be quickly and accurately applied to different neutral grounding methods and types of tree species, providing a new approach for identifying TSF.