Display omitted •1. Single-atom Co modified g-C3N4 prepared by one-step thermal-polymerization of cobalt phthalocyanine (CoPc) and urea.•2. Strong interaction between Co 3d electrons and C 2p electrons of CO2 activate C = O bonds in CO2 molecule.•3. Optimal 1%Co-CN sample exhibits higher CO yield of 94.9 umol/g/h than pure g-C3N4 0.25 umol/g/h. Using solar energy to realize photoreduction of CO2 into valuable chemicals is a potential way to solve energy crisis and carbon cycle. Due to the extremely stable molecular configuration of CO2, activating CO2 molecule is the key and difficult step in the whole CO2 conversation process. In this work, we used density functional theory (DFT) to calculate the reaction pathways of CO2 to CO on pure g-C3N4 and single-atom cobalt (Co) modified g-C3N4. Theoretical calculation predicts that single-atom Co sites modified g-C3N4 (Co-CN) possess stronger CO2 adsorption ability and lower barrier of CO2 hydrogenation activation than pure g-C3N4. The strong interaction between Co 3d electrons and C 2p electrons of CO2 is the crucial factor to activate C = O bonds of CO2 molecule. Better CO2 adsorption and activation abilities also are proved in Co-CN by CO2 adsorption, temperature programmed desorption (TPD), and sensor tests. As a result, the optimal 1%Co-CN exhibits higher CO yield of 94.9 umol/g/h than pure g-C3N4 (0.25 umol/g/h). This work provides a new insight of the role of single-atom sites in CO2 reduction reactions.