The decomposition of cobalt carbide (Co ₂ C) to metallic cobalt in CO ₂ hydrogenation results in a notable drop in the selectivity of valued C ₂₊ products, and the stabilization of Co ₂ C remains a grand challenge. Here, we report an in situ synthesized K-Co ₂ C catalyst, and the selectivity of C ₂₊ hydrocarbons in CO ₂ hydrogenation achieves 67.3% at 300°C, 3.0 MPa. Experimental and theoretical results elucidate that CoO transforms to Co ₂ C in the reaction, while the stabilization of Co ₂ C is dependent on the reaction atmosphere and the K promoter. During the carburization, the K promoter and H ₂ O jointly assist in the formation of surface C ^* species via the carboxylate intermediate, while the adsorption of C ^* on CoO is enhanced by the K promoter. The lifetime of the K-Co ₂ C is further prolonged from 35 hours to over 200 hours by co-feeding H ₂ O. This work provides a fundamental understanding toward the role of H ₂ O in Co ₂ C chemistry, as well as the potential of extending its application in other reactions.