The fundamental insights of the reaction mechanism, especially the synergistic effect between oxygen vacancies and basic sites, are highly promising yet challenging for Ru-based catalysts during carbon dioxide (CO₂) methanation. Herein, a series of Ru-based catalysts were employed to study the mechanism of CO₂ methanation. It is found that Ru/CeO₂ catalyst exhibits a much higher CO₂ conversion (86%) and CH₄ selectivity (100%), as well as excellent stability of 30 h due to the existence of abundant oxygen vacancies and weak basic sites. Additionally, the in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations reveal that the formate formation step dominated the hydrogenation route on Ru/CeO₂ catalyst, and the b-HCOO* could be the key intermediate due to b-HCOO* is more easily hydrogenated to methane than m-HCOO*. The systematic study marks the significance of precise tailoring of the synergistic relationship between oxygen vacancies and basic sites for achieving the desired performance in CO₂ methanation.