Abstract: The thermal properties of the insulating gas in a gas insulated metal-enclosed transmission line (GIL) will affect the overall insulation performance of the GIL. At present, domestic and foreign research on the application of C₄F₇N/CO₂ mixed environmentally friendly gas to GIL has just started, and its thermal characteristics are still unclear. Therefore, we established a temperature-fluid multi-physical field coupling simulation model for the environment-friendly UHV C₄F₇N/CO₂ mixed gas GIL and conducted simulation research. An on-site temperature rise test to effectively verify the model was also conducted. The research results indicate that the internal temperature gradient of environmental-friendly GIL is larger than that of SF₆ GIL, which should be paid attention to in the subsequent design of environmental-friendly GIL. The convection performance of SF₆ is better than that of C₄F₇N/CO₂ mixed gas, which will cause differences of temperature distribution in GIL. The relationship between the temperature rise of GIL center conductor and the top of the C₄F₇N/CO₂ GIL shell is explored, and the GIL conductor-case temperature rise response coefficient is defined. This coefficient can be applied to the subsequent center conductor temperature monitoring system based on the shell temperature.