Efficient　co-catalysts　are　crucial　for　photocatalytic　water　oxidation　towards　conversion　of　solar　energy　into　chemical　energy.　Herein,　we　develop　a　sustainable　and　effective　water　oxidation　system　using　graphitic　carbon　nitride　(g-C3N4)　and　cubic　cobalt　manganese　spinel　(c-CoMn2O4)　as　light　transducer　and　water　oxidation　co-catalyst,　respectively.　The　surface　modified　g-C3N4　with　c-CoMn2O4　not　only　accelerates　the　interface　transfer　rate　of　charge　carriers　but　also　reduces　the　excessive　energy　barrier　for　O-O　formation,　leading　to　an　enhanced　photocatalytic　activity　of　water　oxidation.　Benefiting　from　the　well　surface　engineering　of　g-C3N4,　the　g-C3N4-CoMn2O4　(CN-CM)　composites　exhibit　an　enhanced　performance　of　photocatalytic　water　oxidation.　The　oxygen　evolution　rate　(OER)　of　CN-CM　is　4　times　higher　than　that　of　pristine　g-C3N4.　It　is　a　highly　active　in　oxidation　of　water,　with　an　apparent　quantum　yield　(AQY)　of　ca.　1%　at　380　nm　with　AgNO3　as　sacrificial　agent.　This　improvement　is　mainly　due　to　the　mixed-valence　Co　and　Mn　cations　contained　in　c-CoMn2O4　spinel.