Herein, as an indispensable key reactant for the hydrolysis of organic sulfur, H2O was firstly introduced to optimize the NH3-SCR activity of CeO2 by providing the adsorption and reaction spots of catalysis for organic CS2+COS during the low-temperature gas-phase sulfation. The results demonstrate that the introduction of 0.33 vol% H2O is beneficial to enhance the interaction between organic CS2+COS and cube fluorite CeO2, which not only increases the concentrations of Ce3+ ions, chemisorbed oxygen (Oβ) and oxygen vacancies on CeO2-CS2+COS surface, but also effectively enhances the redox cycle of Ce4+/Ce3+ ion pairs and the medium-strong acid sites of catalyst. These all help enhance the promotional effect of organic sulfur low-temperature gas-phase sulfation on the NH3-SCR activity of CeO2 and further improve NOx reduction over the CeO2-CS2+COS catalyst. However, the introduction of 5.0 vol% H2O shows a certain inhibitory effect due to the competitive adsorption of excess water and CS2+COS on the cube fluorite CeO2 surface, which weakens their interaction during the low-temperature gas-phase sulfation, thereby decreases the promotional effect of low concentration water introduction on the NH3-SCR activity of the CeO2-CS2+COS catalyst. Therefore, the results of this research provide a scientific reference for developing the NH3-SCR CeO2-based catalyst in practical applications.