The frequent occurrence of forest fires in recent years has led to major casualties and significant losses of the national economy, affecting the operational safety of the power grid. Besides lightning, human and other factors, whether the power line tree failure triggered is also a highly controversial issue in the investigation of the cause of the occurrence of forest fires. This paper takes the medium-voltage line as the main research object, a flame and tree-line discharge test platform is built. The fault characteristics of tree discharge near the line, tree discharge due to external factors hitching on the line and line discharge when the flame appears below are studied, and the identification basis of line fault causes by mountain fire or tree is proposed. Finally a theoretical model of tree grounding fault, which can predict the amplitude of electrical parameters such as leakage current is proposed. This study can provide ideas to the prevention of tree line faults in the power grid caused by mountain fire.