In this study, DFT-D calculations were performed to explore the role of Cu and Mo loading in the CO₂ conversion mechanism on a two-dimensional g-C₃N₄(001) surface. The introduced transition metals, Cu and Mo, significantly changed the electron distribution and band structures of g-C₃N₄. Moreover, two possible mechanisms for the reduction of CO₂ to CO have been discussed in detail. We found that the energy barriers of the two mechanisms were largely reduced by Cu and Mo loading, and the dominant reaction path changed on different transition metal-loaded surfaces. Cu/g-C₃N₄(001) prefers to directly dissociate CO₂ into CO, whereas cis-COOH is the preferred product of CO₂ reduction on Mo/g-C₃N₄(001). Considering the activation barrier and reaction route selectivity, Mo-doped g-C₃N₄(001) was identified as a promising candidate for CO₂ conversion. It is concluded that suitable transition metal doping can efficiently reduce the energy barrier and control route selectivity along the reaction paths over the g-C₃N₄ surface. These findings could provide a helpful understanding of the CO₂ reduction mechanisms and aid in the molecular design of novel g-C₃N₄ catalysts for CO₂ conversion.