27.5kV environment-friendly gas insulated switchgear has gradually become a key link in the power supply system of high-speed railway. The heating and heat dissipation of switchgear is a key problem to be studied and solved in the process of its miniaturization, development and application. In this paper, the finite element simulation software ANSYS is used to simulate the environment in the cabinet, and the temperature field and air flow field of 27.5kV gas insulated switchgear are studied. The main research contents include the establishment of three-dimensional model of switchgear based on solid works, the setting of boundary conditions of ANSYS Icepak, and finally solved by fluent solver to simulate the three-dimensional temperature field of switchgear under different insulating gas conditions and different gas mixing ratio. For 27.5kV closed gas switchgear, the temperature field of insulating gas is simulated and compared by using dry air, CF3I gas, SF6 / N2 mixed gas and CF3I / N2 mixed gas respectively. The simulation results lay a foundation for the research and application of environment-friendly mixed gas insulated switchgear from the perspective of temperature field.