Abstract: SF₆/N₂ gas mixture is considered as one of the most promising gases to replace SF₆ because of its good insulation performance and environmental benefits. However, there is a lack of research on the mixing ratio of SF₆/N₂ gas mixture in different conditions. Under the premise of ensuring the insulation level of gas-insulated transmission lines (GIL), a multi-physical field coupling calculation model is established. From the perspective of heat characteristics of gas mixture, the relationship between SF₆/N₂ mixing ratio and GIL temperature rise under different insulating gas pressure, load current and ambient temperature is analyzed to determine the appropriate SF₆/N₂ mixture ratio under different conditions. The results show that there is a negative correlation between the insulation gas pressure (or the SF₆ content) and GIL temperature rise at the same pressure. The SF₆ content is suggested to be in the range of 40%~60% when the load current exceeds 3 kA for a long period of time, 40%~70% when the equipment is running in the middle and low latitudes, and 30%~40% when the equipment is in the high latitudes. In addition, considering the safety margin, the insulation and temperature rise requirements of GIL can be met when SF₆ content is not less than 70% of the recommended value.