In recent years, the gas mixture of perfluoroisobutyronitrile (C 4 F 7 N) has attracted extensive attention as a potential substitute for the strong greenhouse gas SF6 in gas insulation equipment. On this basis, the possibility of C 4 F 7 N/CO 2 /O 2 mixture gas replacing SF6 at medium pressure was studied. The insulation properties such as AC breakdown and partial discharge are systematically studied. It is found that the mixture of 15% C 4 F 7 N/81% CO 2 /4% O 2 has the best dielectric strength and self-recovery performance. Metal protrusion and insulator pollution caused by PD lead to the decomposition of C 4 F 7 N, resulting in by-products CF 4 , CO, C 2 F 6 , C 3 F 8 , and COF 2 .