To address the problem of insufficient and hysteretic determination methods for the identification of overheat faults on gas-insulated transmission lines, fault identification is proposed by monitoring the temperature of the enclosure and using GIL thermal characteristic difference. First, the temperature of the enclosure is studied based on the GIL temperature field. Then the change rate of temperature and its range is defined to be a criterion for describing the difference of thermal characteristics in different states. Finally, a GIL finite element model is established to verify the range of the criterion in different states and the feasibility of temperature change rate as a criterion. Simulation results show that there is a significant difference in the change rate of temperature in different operating conditions. This can accurately identify abnormal operation and short-circuit faults. In addition, this method is applicable to GIL in all lay-ups, and is simple and efficient, providing a basis and methodology for condition assessment and operation and maintenance. This work is supported by the National Key Research and Development Program "Research and Development of Key Technologies and Equipment for Safe Production of the Internet of Mines" (No. 2017YFC0804400), the Science and Technology Program of State Grid Jiangsu Electric Power Co., Ltd. (No. J2019009), and Optimal Dispatch and Protection Configuration of Multi-Power Distribution Network for Pumped Storage in Abandoned Mines (Six Talent Peaks Project).