Wildfires can cause the decrease of breakdown voltage of the air gap of overhead transmission line, which in turn cause the malfunction of overhead transmission line. This paper analyzes three reasons for the decrease of breakdown voltage of the air gap caused by wildfires, including high temperature, floating objects, electrons and ions in the flame. Based on these three factors, the air gap characteristics of the wildfires malfunction of the transmission line are divided into flame zone, ion zone and smoke zone. Furthermore, this paper analyzes the characteristics of transmission line malfunction caused by wildfires in North China, including the time distribution, combustion characteristics, wind speed and flashover path of malfunctions. In order to improve the ability to resist the wildfires, technical means and measures for preventing malfunction caused by wildfires are proposed, including strictly controlling the fire source around the transmission line corridor, removing the combustible material in time, and transferring the load of the transmission line reasonably. This work lays a foundation for improving the protection of overhead transmission lines against wildfire.