Abstract: Silicon carbide/zirconium diboride (SiC-ZrB₂) ultra-high temperature ceramics (UHTCs) are commonly employed for sharp components of hypersonic vehicles under extreme thermal environments to maintain geometric configuration. The key mechanism lies in the formation of a dense solid-liquid mixed oxide scale (with pores filled by borosilicate glass) during high-temperature oxidation, which effectively impedes oxygen diffusion and achieves near-zero/minimal ablation. Based on oxygen diffusion mechanisms in porous oxides and thermochemical equilibrium principles, a criterion for SiC depletion layer formation was proposed, establishing a dual-model system accounting for both presence and absence of depletion layers. For cases with depletion layers, the model couples CO/CO₂ convective diffusion, ZrO₂ solid layer growth, and the competitive mechanisms of B₂O₃-SiO₂ glass layer evaporation/growth to quantify virgin material recession and weight gain. For cases without depletion layers, recession equations for substrate interfaces and critical composition conditions were derived. Numerical calculations identified that optimal oxidation resistance occurs at 20—30 vol% SiC content, where oxide products exhibit maximum density. Model validation conducted in static air environments at 1473, 1573, and 1773 K demonstrated good agreement between calculated oxidation weight gain curves and experimental data, confirming the model's capability to accurately simulate the influence of SiC content variations on oxidation behavior. This model effectively simulates the oxidation behavior of SiC-ZrB₂ ceramics and provides critical evaluation criteria for oxidation resistance in material optimization design.