Graphene　as　an　electron-conductive　medium　has　been　widely　used　to　construct　efficient　photocatalysts　for　solar　energy　conversion,　while　the　discovery　of　photosensitizer　role　of　graphene　further　enriches　our　understanding　of　the　multifunctional　facets　of　graphene　in　graphene-based　composite　photocatalysis.　However,　the　photosensitive　efficiency　of　graphene　remains　relatively　low,　and　the　photosensitization　mechanism　has　yet　to　be　elucidated.　Herein,　we　report　a　facile　wet　chemistry　strategy　to　enhance　the　photosensitive　efficiency　of　graphene　via　oxidation　treatment　of　graphene　oxide　and　particularly　reveal　the　underlying　origin　of　such　boosted　photosensitive　efficiency　of　graphene.　Graphene　derived　from　graphene　oxide　with　enhanced　oxidation　degree　shows　remarkably　improved　photosensitive　efficiency　for　wide-bandgap　ZnO　toward　visible-light-driven　photoreduction　process.　Controlled　experiments　disclose　that　the　residual　content　of　oxygenated　functional　groups　is　the　main　factor　affecting　the　photosensitive　efficiency　of　graphene.　Furthermore,　theoretical　calculations　indicate　that　increasing　the　content　of　oxygenated　functional　groups　in　graphene　is　able　to　widen　the　bandgap　of　graphene　with　the　upshift　of　its　conduction　band,　which　endows　the　electrons　in　the　photocatalytic　system　with　enhanced　reduction　capacity　and　thus　boosts　the　photosensitive　efficiency　of　graphene.　These　encouraging　results　would　provide　instructive　guidelines　for　the　utilization　of　multifunctional　roles　of　graphene　toward　designing　advanced　graphene-based　composite　photocatalysts　for　solar　energy　conversion.