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As a new environmental-friendly gas insulating medium, perfluoroisobutyronitrile (C4F7N) has been widely concerned in recent years due to its relatively low global warming potential and excellent dielectric strength, which has the potential to replace the most greenhouse gas, sulfur hexafluoride(SF6). However, there are a few systematic studies on the influence of gas pressure and mixing ratio on the dielectric strength of C4F7N/CO2 gas mixture at present. In this paper, the power frequency breakdown characteristics of C4F7N/CO2 gas mixture under different pressure and mixing ratio conditions were tested using the gas insulation performance test platform. The optimal mixing ratio and pressure range of C4F7N/CO2 gas mixture for engineering application were also discussed. It is found that the breakdown voltages of C4F7N/CO2 gas mixture show a saturated growth trend with gas pressure and mixing ratio. The breakdown voltage of the gas mixture with 10% C4F7N can reach 80% of pure SF6 under the same condition. The insulation performance of the gas mixture with 20% C4F7N can reach more than 95% of pure SF6. Relevant results indicate that the gas mixture with 4%-12% C4F7N has the potential to be applied to high-voltage gas-insulated equipment.