C 5 F 10 O as a new type of environmental protection gas insulation medium has attracted extensive attention. Its switching impulse breakdown characteristic is an important element for engineering. In this paper, the saturated vapor pressure characteristics of C 5 F 10 O/CO 2 mixed gas at different mixing ratios are calculated firstly by combining the Antoine vapor pressure equation and the basic law of vapor liquid equilibrium. Secondly, the needle-plate electrode structure is used to simulate the extremely non-uniform electric field. The influence of gas pressure, mixing ratio, and voltage polarity on the insulation characteristics of the mixed gas is analyzed. Finally, the synergistic effect of the mixed gas is analyzed. The results show that the polarity effect of C 5 F 10 O/CO 2 mixture under the switching impulse voltage is obvious in the extremely non-uniform electric field. When the voltage is negative, the $U_{50\%}$ of the mixed gas increases in a linear fashion with the increase of the gas pressure. The breakdown voltage under positive polarity appears “hump”. The breakdown characteristics of mixed gas improve obviously with at the lower mixing ratio and little variation at the higher mixing ratio. The synergistic effect between the components of the gas mixture is also affected by gas pressure and C 5 F 10 O content.