Coal ash and water vapor are usually present in the flue gas derived from coal combustion, which inevitably impact the performance of calcium sorbents in a Calcium-Looping process for CO2 capture. This study investigated the influence of high-temperature water vapor and coal ash on the carbonation behavior of gamma-Al2O3 supported calcium sorbent in a fluidized-bed reactor and a TGA apparatus. It was found that the presence of water vapor during carbonation induced the extensive formation of large voids on the sorbent surface and the decomposition of inert mayenite (Ca12Al14O33) into CaO and Al2O3. As a result, the reactivity and carrying capacity of the synthetic sorbent were significantly increased. During the significant exothermic carbonation reaction, the enhanced cooling effects of sorbent particles by water vapor in flue gas may provide a better equilibrium condition for higher carbonation rate and capture capacity. Coal ash impurities caused severe decay of the sorbent surface, but high-temperature water vapor exerted a very strong activation function to the ash contaminated synthetic sorbents for better carbonation.