Abstract: The effects of H₂O on the performance of the sulfur-resistant Mo-based methanation catalysts were investigated by adding vapor into the reactant gas at the reaction conditions of 550℃, 5 500 h^-1, 1.2% of H₂S concentration. The results indicate that water caused irreversible deactivation of Mo-based catalyst supported on Al₂O₃, while additive Co and cerium-aluminum composite carrier can enhance the activity and improve the stability of the Mo-based catalyst for methanation of syngas. The promoter Co protects the active phase MoS₂ of Mo-based catalyst, inhibits the irreversible deactivation caused by the addition of water. When the water content in reactant gas is increased, the water gas shift reaction increases and becomes the main reaction on catalysts, and the increase of water leads to further decrease of the catalyst activity and stability.