Tree-related failure is a typical non lightning fault on transmission lines. Current researches mainly focus on the diagnosis and treatment, while the mechanism study is seldom reported. Firstly, the formation process of tree-related failure and the typical discharge characteristics in different stages of tree height are analyzed, and then different phases of tree-wire gap discharge process are simulated in the lab environment. The discharge current is monitored, and the characteristics of the waveform and the frequency spectrum are analyzed. Characteristics and rules of tree-wire discharge at different stages are obtained, and the mechanism of transmission line tree-related failures is further expounded.