The electrocatalysts containing cobalt-pyrrolic nitrogen-carbon (Co-N₄-C) moiety for CO₂ reduction reaction (CO₂RR) have caught much attention. However, the effects of Co valence state and its synergy with graphene substrate are not clear yet. In this work, cobalt porphyrin (CoTPP) molecule with the intrinsic Co-N₄-C moiety is successfully combined with graphene oxide (GO) via three kinds of liquid-phase methods. The ratio of CoTPP to GO and the valence state of Co atom are studied to explore their catalysis for CO₂RR to CO. It is found that axially-coordinated Co(III)TPPCl/GO nanocomposites synthesized via a chemical method exhibit better ability for CO₂RR, as compared with Co(II)TPP+GO and/or Co(III)TPPCl+GO nanocomposites obtained via a physically mixing way. After optimizing the ratio of CoTPP to GO, the Faradaic efficiency (FE) is more than 90% for CO₂RR to CO between −0.7 and −0.8 V vs. reversible hydrogen electrode (RHE) in Co(III)TPPCl/GO75. The synergy between CoTPP and GO and the effect of Co valence state are systematically investigated, indicating that their strong interaction plays the key role in electrocatalytic CO₂RR.