The dielectric strength of the transmission lines gap has a significant decline under the wildfire, then leading to the frequent lines’ tripping accidents, which has a serious effect on the safe and stable operation of the grid. To analyze the decline cause of the transmission lines gap’s dielectric strength, three kinds of typical vegetations which can easily make the line tripping were selected in this paper; the dielectric strength of the vegetations flame were measured, and the influence mechanism of the vegetation flame’s dielectric strength had been analyzed. The results show that, the resistivity of the three vegetations flame is small under the large burning intensity. Among them, fir branch is minimum, for 3 kΩ?m, and fir body is largest, reached to 30 kΩ?m, the dielectric strength of which are lower than pure air gap. The dielectric strength of flame decreases with the increase of flame height and increases with the reduction of flame height. The insulating properties of vegetations flame are related with the flame height, temperature, smoke concentration, ash content and other factors, while the flame height and burning intensity have a greater impact on the insulation characteristics of vegetations flame when the temperature is lower. Hence, controlling the height and density of vegetations in the transmission line corridor can reduce the accidents of line tripping under wildfire radically.