With the growth of power demand and the increase of environmental protection requirements, the use of SF₆ gas is gradually limited. SF₆ mixed gas reduces the amount of SF₆ gas to a certain extent and has been used in electrical equipment. The insulation change caused by SF₆ mixed gas leakage in 220 kV gas insulated switchgears (GIS) is studied. By changing a small amount of pressure and mixing ratio, the insulation performance change of the mixed gas is explored, and the influence law of pressure and mixing ratio factors on the breakdown voltage of power frequency is analyzed. The insulation strength curves of mixed gases in different proportions at each pressure are obtained, so as to obtain the air supplement strategy to ensure the safe and stable operation of the equipment. It is found that the breakdown voltage of mixed gas changes with the increase of pressure and mixing ratio, and the nonlinear degree increases, and the boundary values of mixing ratio and pressure for safe operation of equipment are obtained. The study can provide operation and maintenance procedures and technical standards for SF₆/N₂ mixed gas insulation equipment. At the same time, it lays the foundation for the development of mixed gas detection technical standards.