In this paper, a well-sealed experimental chamber with adjustable electrode spacing is designed to simulate the slightly inhomogeneous field in high-voltage electrical equipment by adjusting the appropriate contact spacing. The research focuses on the C4F7N/CO2 mixture, with SF6 and CO2 gases serving as controls for the experiment. The total pressure was increased by adding the buffer gas CO2 in the experiment. The effects of partial pressures, total pressures and other factors on the insulation characteristics of the gas mixture are systematically investigated through the power frequency breakdown experiment. A comparative analysis of the advantages and disadvantages of the new SF6 replacement gas and SF6 is conducted, and discusses its potential as a substitute for pure SF6. It reveals that the C4F7N/CO2 gas mixture has a great potential to replace SF6.