Calcium cycling is a second-generation CO2 capture technology characterized by rapid development and high technical maturity. In practical applications, steam and SO2 are commonly present in the calcination reactor due to the oxygen-rich combustion of coal. Previous studies have shown that steam accelerates the decomposition of CaCO3 and shortens the calcination time. However, steam also significantly enhances sulfation in the presence of SO2, and there remains no consensus in the literature regarding the combined effects of steam and SO2 on the decomposition of CaCO3. This study investigates the decomposition characteristics of CaCO3 over time under various reaction atmospheres. The results indicate that while steam accelerates the decomposition of CaCO3, the presence of SO2 almost neutralizes the positive effect of steam. Notably, steam promotes rapid sulfation during the early stages of calcination, which subsequently inhibits the decomposition of CaCO3. Physicochemical characterization reveals that the simultaneous presence of steam and SO2 leads to the rapid formation of CaSO4 on the sorbent surface, with steam-induced pore expansion further enhancing sulfation in the internal pores of the grains. Additionally, this study elucidates the mechanism by which steam accelerates CaCO3 decomposition. It was found that the presence of steam increases the adsorption energy of the CaCO3 (1 0–1 4) surface for SO2 and that H2O molecules facilitate a lower activation energy for the formation of HSO3* compared to SO3*. These findings validate the experimental results and provide a deeper understanding of the combined effects of steam and SO2 during the calcination process from a microscopic perspective.