CF3SO2F (trifluoromethanesulfonyl fluoride) is a novel substitute to SF6. The AC breakdown tests of pure CF3SO2F, CF3SO2F/N-2, and CF3SO2F/CO2 mixtures in a slightly nonuniform electric field were done. The effects of pressure, electrode distance, and mixing ratio on the breakdown characteristics of the CF3SO2F mixtures were obtained and compared with SF6. In order to investigate the micromechanism of dielectric properties, the structures of CF3SO2F, N-2, and CO2 and their bimolecular complexes were determined based on density functional theory (DFT). The differences in dielectric properties between the CF3SO2F/N-2 and CF3SO2F/CO2 mixtures were analyzed. The results show that the breakdown strength of the 40%CF3SO2F/N-2 and 45%CF3SO2F/CO2 mixtures are equivalent to SF6, whereas the CF3SO2F/N-2 mixture is better than the CF3SO2F/CO2 mixture. The reason is that both the ionization energy and the enhancement to the most easily dissociated 1S-2C chemical bond of CF3SO2F center dot center dot center dot N-2 complex are greater than those of the CF3SO2F center dot center dot center dot CO2 complex, which helps the CF3SO2F/N-2 mixture to maintain a higher insulation level.