In　artificial　photocatalysis,　sluggish　kinetics　of　hole　transfer　and　the　high　recombination　rate　of　charge　have　been　the　Achilles’　heel　of　photocatalytic　conversion　efficiency.　The　development　of　co-catalysts　for　promoting　exciton　splitting　and　charge　separation　has　therefore　gained　continuous　attention.　Herein,　“Co4O4”　cubane　complex　Co4O4(O2CMe)4(py)4　(1a,　py　=　pyridine　derivatives),　serves　as　a　homogeneous　molecular　co-catalyst　which　is　engineered　into　the　framework　of　pristine　polymeric　carbon　nitride　(PCN)　for　water　oxidation　with　light　illumination.　This　modification　strategy　maximizes　the　contact　areas　between　co-catalysts　and　reactants,　reduces　the　over-potential　of　oxygen　producing　and　accelerates　the　interface　transfer　rate　of　electron-hole　pairs,　thus　leading　to　enhanced　reaction　kinetics　for　photocatalytic　water　oxidation.　Compared　with　pristine　PCN,　the　photocatalytic　O2　production　rate　up　to　19-fold-enhanced　in　the　presence　of　molecular　co-catalyst.　This　work　emphasizes　the　importance　of　developing　effective,　stable　and　earth-abundant　molecular　co-catalysts　for　the　promotion　of　water-splitting　photocatalysis.　©　2018