To investigate the insulation performance of the novel C4F7N-based ternary gas mixture from a microscopic perspective, the ionization and attachment characteristics of mixtures using CF3SO2F, C4F7N and buffer gases were measured. A series of steady-state Townsend(SST) experiments were conducted on the potential C4F7N ternary mixtures. The results indicate that, with a constant fraction of CF3SO2F, increasing the concentration of C4F7N from 6% to 10% enhances the reduced attachment coefficient η/N, while decreasing both the reduced ionization coefficient α/N and the reduced effective ionization coefficient (α-η)/N. Comparative analysis of C4F7N ternary mixtures with different buffer gases reveals that the α/N coefficients of N2- and CO2-containing mixtures are similar, whereas the η/N coefficients differ significantly. However, as the content of C4F7N increases, the critical electric field (E/N)c of ternary mixtures with CO2 as buffer gas are lower than those with N2 as buffer gas, indicating N2 is superior to CO2 as the buffer gas for C4F7N ternary gas mixture. The presence of excited-state molecules in N2 reduces the density of high-energy electrons, thereby enhancing electron attachment of C4F7N and CF3SO2F. Under the constraint of liquefaction temperature and insulation strength, the mixture of 20% CF3SO2F/10% C4F7N/70%N2 is selected. It exhibits the highest electric field strength, reaching 94% of that of SF6, demonstrating excellent potential for isobaric replacement of SF6 at the same pressure.