Abstract: The microcosmic mechanism of the accelerating effect of water on CaO sulfation reaction was studied by the experiment and density functional theory. The results show that the addition of water in the slow reaction stage immediately increases the sulfation rate. The energy barrier of the formation of CaSO₄ product layer on CaO surface is smaller than that of H₂O dissociation, which may be the reason that water has no acceleration effect in the rapid reaction stage. In the anhydrous condition, the control step in the slow reaction stage is not the diffusion of solid-state ions in CaO and CaSO₄ crystals, but the diffusion process of calcium ions on the original CaSO₄ surface to the newly formed CaSO₄ product layer, whose energy barrier is 3.61eV. In the hydrous condition, after the H₂O molecule adsorbs on the CaSO₄ surface and dissociates into H^* and OH^* with an energy barrier of 2.91eV, the energy barrier of the control step of anhydrous condition is reduced by 1.98eV, and the rate-limiting step for the hydrous condition changes to the diffusion of O^2- in the CaSO₄ crystal, whose energy barrier is 0.19eV lower than the rate-limiting step of the anhydrous condition.