Carbon-fiber-reinforced carbon and silicon carbide (C/C-SiC) composites were prepared using chemical vapor infiltration (CVI) combined with reactive melt infiltration (RMI). The microstructure and flexural properties of C/C-SiC composites after oxidation in different temperature water vapor environments were studied. The results indicate that the difficulty of oxidation in water vapor can be ranked from easy to difficult in the following order: carbon fiber (CF), pyrolytic carbon (PyC), and ceramic phase. The surface CFs become cone-shaped under corrosion. PyC has a slower oxidation rate and lower degree of oxidation compared to CF. The SiO₂ layer formed by the oxidation of SiC and residual Si was insufficient to fully cover the surface of CFs and PyC. As the temperature increased, the oxide film thickened, but the corrosion degree of CF and PyC intensified, and the flexural performance continuously deteriorated. The flexural strength of C/C-SiC composites was 271.86 MPa at room temperature. Their strength retention rates were all higher than 92.19% after water vapor corrosion at 1000 °C, still maintaining the “pseudoplastic” fracture characteristics. After water vapor corrosion at 1200 °C, the CFs inside the composites sustained more severe damage, with a strength retention rate as low as 48.75%. The fracture mode was also more inclined towards brittle fracture.