Abstract: A reaction bonded SiC ceramic used in a suspension preheater inner cylinder was comparatively analyzed before and after use by X-ray diffractometer,scanning tunneling microscopy and energy dispersive spectroscopy,and its damage mechanism was studied. The results show that residual silicon metal in the Si C ceramic and silicon carbide in the surface are firstly oxidized into SiO₂ under the high temperature atmosphere and the liquid viscosity of SiO₂ decreases,resulting in the formation of an oxidation layer because of the existence of an alkaline steam of K₂O(g),Na₂O(g),KCl(g) and NaCl(g) and chlorides. Afterwards the oxidation layer is scoured and frayed by high-speed air flow,leading to a new interface. With the cycle repeating,the outside of the Si C ceramic becomes to be thinner little by little and appears to rupture till damage. As a result,the promotion of densification and the reduction of residual silicon metal are effective ways to improve the use ability of the reaction bonded Si C ceramic in a suspension preheater.