Abstract: Overhead transmission lines may trip and shut down under the influence of forest fire, which seriously threatens the safe and stable operation of power grid. At present, the influence of altitude is not taken into account in the study on the reduction of air gap insulation strength by simulating forest fire, and there is no basis for the evaluation of insulation strength of transmission lines under forest fire conditions in high altitude areas. The differences of burning characteristics and air gap breakdown characteristics of Chinese fir crib flame at altitudes of 23 m and 2 013 m were studied by using a simulated forest fire test platform in this paper. The results show that the flame temperature changes linearly with height, and the flame center temperature at high altitude is 18.3% lower than that at low altitude. The discharge arc channel is thicker and the temperature is higher at low altitude, and the voltage required for pre-discharge at high altitude is lower. The breakdown voltage gradient of the main part of the flame at two typical altitudes has a negligible difference, with an average of 60.5 kV/m. The main part of the flame does not need altitude correction. The influences of air density, charged particles and particulate matter should be considered when correcting the altitude of the non-main part of the flame. The decrease of air density is the main factor, and the higher the altitude, the greater the influence of particulate matter. The research results can provide references for forest fire prevention and control of transmission lines in high altitude areas.