With the advantages of maximum metal utilization, single-atom catalysts (SACs) are emerging as promising catalysts in the CO₂ reduction reaction (CO₂RR) field. Herein, first-principles calculation is performed to investigate the CO₂-to-CO conversion mechanism by a serial of transition metal single transition metal atom supported on graphene systems (TM@Gr_s) as a CO₂ reduction catalyst. Among all SACs considered in this work, Cr@Gr_s exhibits a low limiting potential of −0.21 V, showing remarkable performance for CO₂RR. To understand activity origin of CO₂RR, we analyzed the effect of d-band centers of TM@Gr_s and investigated the charge transfer and bonding/antibonding states between the intermediates and TM atoms. These physical quantities provide a good explanation for the process of CO₂RR and show the theoretical guidance for proper catalyst discovery and better carbon circulation.