Sulfur hexafluoride (SF6), widely used in high-voltage switchgear, possesses a strong greenhouse effect, prompting an urgent need for environmentally friendly alternatives. Among them, C4F7N/CO2 gas mixtures are considered highly promising. As environmentally friendly power equipment is developed for higher voltage levels, the insulation performance of gas mixtures with low C4F7N content requires further investigation. This study explores the power frequency breakdown characteristics of low-content C4F7N gas mixtures under different electric field uniformities. The results show that the power frequency breakdown voltage exhibits a strong dependence on the electric field distribution. Under highly inhomogeneous fields, the gas mixtures display a nonlinear saturation effect in breakdown voltage, with the pressure-dependent increase reduced by 30%–50% compared to that under slightly inhomogeneous conditions. In slightly inhomogeneous fields, adding 4%–8% O2 improves insulation performance by up to 9.35%. However, under highly inhomogeneous conditions, increasing the O2 concentration to 12% is required to enhance the dielectric strength effectively.