At present, the change characteristics of the proportion of each component after the leakage of C4F7N/CO2/O2 ternary mixed gas in GIS equipment in operation are not clearly, which can not guide the leakage accounting of ternary mixed gas. In this paper, the leakage model of sealing ring is built firstly. Then, the leakage of C4F7N/CO2/O2 ternary mixed gas with the component ratio of 8.5%:86%:5.5% was estimated at different annual leakage rates. Next, the experimental parameters was determined and the simulation experimental platform of C4F7N/CO2/O2 ternary mixed gas leakage under pressure is built. Finally, combined with the synchronous leakage method of multiple leakage points, the leakage experiments in vacuum and atmospheric pressure was simulated respectively. The results show that under the three annual leakage rates of 0.5%/year, 5%/year and 50%/year, the proportion of each component of ternary mixed gas leaked into vacuum environment has little deviation from the proportion of source gas, which can be regarded as equal proportion leakage. Under air condition, when the pressure of ternary mixed gas is higher than 0.4 MPa, the deviation between the proportion of each component and the proportion of source gas is small, which is an equal proportion leakage. And, when the pressure of ternary mixed gas is lower than 0.4 MPa, the proportion of each component deviates greatly from the proportion of source gas, which is a non-proportional leakage. Experiments show that the leakage of C4F7N/CO2/O2 mixed gas, which is in the GIS equipment (rated pressure is higher than 0.8 MPa), can be regarded as proportional leakage.