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.　©　2017