Abstract: Thermal protection materials for reusable high-speed aircraft suffer from coupled aerodynamic heating and oxidation damage. Conventional arc-heated wind tunnel tests cannot simultaneously reproduce the surface heat flux and near-wall oxygen atom concentration of flight conditions which limits the accuracy of ground-based evaluation of flight conditions. To address this, we numerically solve the high-enthalpy thermochemical non-equilibrium flow equations for arc-heated wind tunnels with varying nitrogen-oxygen mass ratio gas mixtures, and combine the simulations with wind-tunnel experiments to analyze the effects of nitrogen-oxygen mass ratio on the flowfield and thermal environment around the stagnation model. It is found that, under identical total enthalpy and total pressure, varying the nitrogen-oxygen mass ratio has a minor effect on the flowfield structure, pressure, and velocity. The variation in cold-wall heat flux on the model surface is negligible, with only the proportion of conductive and diffusive heat flux being different, and the variation in hot-wall heat flux is insignificant. In contrast, the nitrogen-oxygen mass ratio significantly influences the oxygen atom mass fraction in the flowfield. Specifically, when the nitrogen mass fraction in the test gas is increased from 0.767 to 0.90, the cold-wall heat flux near the stagnation changes by less than 5%, the hot-wall heat flux increases by a maximum of approximately 8%, and the oxygen atom mass fraction near the surface decreases by approximately 57%. The conclusions indicate that we can effectively coordinate and approximate both surface heat flux and oxygen atom concentration, by rationally adjusting the nitrogen-oxygen mass ratio of the test gas in an arc-heated wind tunnel. This approach provides an important reference for the repeated thermal assessment and service life evaluation of thermal protection materials.