High-temperature oxidation-induced degradation is a critical factor limiting the application of C/SiC ceramic matrix composites in aerospace thermal-structural components. To accurately predict the oxidation damage evolution and mechanical property degradation of such materials, a systematic numerical study is conducted in this work. First, based on Fick’s diffusion law and the characteristics of oxidation reactions, chemical consumption terms and damage factors are introduced to establish a coupled diffusion–reaction oxygen concentration equation and an oxidation state evolution model. Using the COMSOL platform, the oxygen concentration field and oxidation damage factor distributions of a two-dimensional woven C/SiC composite subjected to oxidation at 650 ℃ for 2 h, 4 h, and 6 h are simulated, revealing the spatial-temporal distribution and evolution of oxygen in different structural regions. Subsequently, by integrating continuum mechanics with progressive damage theory, a coupled stiffness-degradation model incorporating limited and continuous degradation is developed, and the post-oxidation tensile response is predicted using the ABAQUS platform. The simulation results show good agreement with experimental data in terms of stress–strain behavior and residual strength, with a maximum error below 10%. This study provides theoretical support and numerical tools for evaluating the postoxidation mechanical performance of C/SiC composites under high-temperature service conditions.