Metal–support interactions strongly affect the catalytic performances of ceria-supported metal catalysts, and hydrogen treatment at high temperature is important for the preparation of catalysts that show strong metal-support interaction (SMSI). With SMSI, the proportion of metal species existed in the form of metallic state is lowered, consequently hindering the performance of a metal catalyst for a reaction that requires metallic sites. Here we show that CO activation of a Ru/CeO2 catalyst not only enhances the reduction degree and exposure of Ru species, but also increases Ce3+ concentration, oxygen vacancy (OV) and active oxygen, resulting in the formation of electron-enriched Ruδ− species and Ruδ−−OV−Ce3+ sites. As a result, a Ru/CeO2 catalyst after CO activation shows high ammonia synthesis activity, and the ill effect of hydrogen poisoning is effectively alleviated. These findings are important for the design of supported metal catalysts that afford metallic species as active sites.