A tree-contact single-phase-to-ground fault of a transmission and distribution line corridor may ignite vegetation, cause fire, and threaten the normal operation of the line. Vegetation moisture content is an important factor affecting the single-phase grounding fault of a conductor contact tree, but its influence on the fault development process and fault characteristics is still unclear. In this paper, a single-phase ground fault test is carried out on a typical vegetation- eucalyptus. Based on the test phenomenon and leakage current waveform, the development process of vegetation high resistance grounding fault is analyzed from the perspective of carbonization channel development and moisture content change. The influence law of vegetation moisture content on leakage current, carbonization path, equivalent resistance and other characteristics during the fault process is studied. Then, the relationship between the first peak value of leakage current and water content is revealed, and a prediction model of the first peak value of leakage current based on vegetation length, diameter and water content is established. The accuracy of the prediction model is verified by experiment. The results of this study are of significance to the detection, diagnosis and prediction of a single-phase ground fault of a wire contact tree.