In order to improve the service performance and explore the damage mechanism of silicon carbide-mullite bricks for the transition zone of cement rotary kilns, the phase composition and the microstructure of a used brick in the transition zone of a cement rotary kiln were analyzed by XRD, SEM and EDS. The results show that the liquid and alkali vapor phases generated by the reaction between cement materials and the silicon carbide-mullite brick mostly enter the silicon carbide-mullite brick through the pores; meanwhile, Ca+ and K+ in the cement penetrate through the liquid maintaining a high chemical potential energy to dissolve Al2O3 and SiO2 at the top of the liquid phase thus enhancing the phase penetration; with the decreasing temperature, crystals such as gehlenite, potassium feldspar and potassium chloride are precipitated, which destroy the original structure and increase the difference of thermal expansion coefficient between the high temperature dense end and the metamorphic layer thus resulting in cracks, spalling, and rupture.