Capturing SO2 using natural limestone in situ is an important way of desulfuration in a circulating fluidized bed (CFB) boiler. The limestone desulfuration mainly consists of two processes in the condition of standard pressure and air atmosphere: calcination and sulfuration. Water vapor in the flue gas influences the calcination, and the physical characteristics such as the pore structure of calcined CaO play an important role in the following sulfation. The impacts of water vapor during calcination may be reflected on the surface micromorphology and pore structure of calcined CaO. This work aims to understand the influence of water vapor on surface morphology and pore structure of calcined CaO. One kind of Chinese limestone was used to study the issues in a rotatable fluidized bed reactor. Scanning electron microscope (SEM), confocal scanning laser microscope (CSLM), mercury injection apparatus (MIP), and N-2 adsorption instrument were employed to test the micromorphology and pore structure of calcined CaO. Results show that the existence of water vapor accelerates calcination and shortens the reaction time, but higher water vapor content results in slightly lower ultimate degree of conversion. Testing results of SEM and CSLM show that water vapor improves sintering and growth of grains of calcined CaO. The change of surface roughness also proves the above conclusion; besides, the results of MIP and N-2 adsorption instrument show that there are pores with a size range of 15-80 nm inside calcined CaO without H2O(g), and after sintering under the condition of H2O(g), they will combine together to form relatively bigger pores with a size range of 40-100 nm. As a result, the average pore size increases and the specific surface area decreases but the specific pore volume is less influenced. The fractal dimensions of pore structure for calcined CaO under different concentrations of water vapor were calculated using the data of N-2 adsorption. Results show that as the concentration of water vapor increases, the fractal dimension first decreases and then slightly increases but is still lower than that without H2O(g). It means that the pore structure of calcined CaO becomes simpler with the impact of water vapor, which is beneficial for the reaction between calcined CaO and gases such as SO2.