The oxidation resistance of high-temperature ceramic thermal protection materials such as silicon carbide (SiC) is one of the key factors determining the thermal safety of hypersonic vehicles (Mach > 5). However, the oxidation resistance of silicon carbide is highly sensitive to environmental conditions. Particularly in high-temperature humid environments, its oxidation mechanism remains elusive, representing a critical challenge that must be urgently addressed in the design of future thermal protection systems utilizing transpiration cooling technology. In this work, reactive force field (ReaxFF) molecular dynamics simulations were employed to systematically investigate the atomic-scale oxidation mechanisms of the 6H-SiC (0001) surface under different H₂O/O₂ mixed environments at 1500 K. The results reveal a pronounced two-stage influence of water on the oxidation process of SiC. At the initial stage, H₂O reacts with adjacent adsorbed O₂ molecules on the surface to form hydroperoxyl (-OOH) intermediates. This reaction pathway significantly reduces the dissociation energy barrier of O₂ from 0.78 eV in a pure oxygen environment to 0.35 eV, leading to an earlier onset of O₂ dissociation and promoting the initiation of surface oxidation. As the reaction proceeds, the SiC surface becomes progressively occupied by oxygen atoms and hydroxyl groups. Owing to the much stronger adsorption of hydroxyl groups at unsaturated silicon sites (with an adsorption energy of −3.5 eV compared with −1.13 eV for O₂), the adsorbed hydroxyl groups passivate these active sites and inhibit subsequent O₂ adsorption. Meanwhile, the presence of H₂O reduces the effective concentration of O₂ in the system. The combined effects result in a lower oxidation rate during the oxidation stage compared with that under pure oxygen conditions. Overall, this study elucidates the mechanistic role of water in the high-temperature oxidation behavior of SiC at the atomic scale, providing theoretical insight into the design of thermal protection systems for hypersonic vehicles operating in complex environments.