Graphite is an important structural material and moderating material in high temperature gas-cooled reactor (HTGR). Improving the oxidation resistance of graphite is an important guarantee to improve the safety of HTGR. SiC coating is one of the most widely used coating materials at present. In this paper, a dense SiC antioxidant coating was prepared by fluidized bed chemical vapor deposition process on matrix graphite sphere. And its oxidation behavior and mechanism were analyzed at 1400℃. The results show that the atmosphere has a significant effect on the oxide layer thickness. In the case of single steam, the thickness of the oxide layer obtained is thinner than that in the mixed atmosphere. The surface oxidized by single steam has a smoother surface, slighter point cracks and smaller linear cracks than that oxidized by mixed atmosphere. The oxidation of dense SiC coatings occurs on the surface without destroying the structure of the dense layer. The oxidation mechanism analysis shows that the surface morphology, crack formation and thickness change of the dense SiC coating are all related to the oxidation competition reaction and oxidation intermediates. In the oxidation process of dense SiC coatings in different environments such as air and water vapor, a certain thickness of SiO2 self-healing layer can be formed on the surface. After 10 hours of oxidation, the oxidation depth of the coating is only within 3.5 μm of the surface, and the structure of the dense layer is not destroyed.