In pursuit of dual carbon reduction objectives and the advancement of sustainable transmission infrastructure, the substitution of SF6 with C4F7N has garnered significant interest in the power sector and academia. This study focused on the application of C4F7N/CO2 gas mixtures in 126 kV pneumatic circuit breakers. Using established gas properties, a magnetohydrodynamic-based arc model simulates interruption processes under 0.6 MPa and 40 kA short-circuit currents. Comparative analyses were conducted for various gas compositions, including 6%C4F7N/94%CO2, 9%C4F7N/91%CO2, 15%C4F7N/85%CO2, and pure SF6. The study evaluates key arc parameters: temperature, voltage, and pressure within the arc extinction chamber. The Mayr arc model assesses post-arc thermal interruption performance. Results show mixed gases offer lower arc core temperatures and improved axial diffusion, albeit with reduced radial diffusion and weaker gas blowing effects compared to SF6. Mixed gases also exhibit lower arc voltages. Notably, the thermal interruption capability of 6%C4F7N/94%CO2 is 34.03% of pure SF6, while 15%C4F7N/85%CO2 achieves 57.14%. Increasing C4F7N content in C4F7N/CO2 mixtures enhances thermal interruption performance, highlighting opportunities for developing environmentally friendly high-voltage circuit breakers.