SF_6 is a widely used as gas insulating medium in electrical equipment at present, but its greenhouse effect has a great impact on the environment. Therefore, it is of great significance to study the environmental friendly gas insulating medium which can replace SF_6. CF_4 is a strong electronegative gas with low global warming potential (GWP) index and liquefaction temperature. In this paper, the power frequency breakdown characteristics of CF_4/CO_2 mixture were studied by power frequency breakdown test, and its synergistic effect, GWP value are also analyzed. The results show that the insulation strength of CF_4/CO_2 mixture with 50% CF_4 mixing ratio can reach about 90% of pure CF_4, and the GWP value is low, so it has potential for gas insulation. The synergistic effect of CF_4/CO_2 mixture is between 0.12 and 0.38, showing obvious synergistic effect.