The　fluorescence　(FL)　activity　of　graphitic　carbon　nitride　nanosheets　(CNNSs)　is　dependent　on　both　their　band　gap　and　surface　defect　states.　It　is　known　that　the　band　gap　of　CNNSs　is　difficult　to　tune;　however,　studies　on　the　regulation　of　the　FL　activity　of　CNNSs　by　surface　defect　states　are　still　few.　Herein　a　facile　strategy　to　enhance　the　FL　activity　of　CNNSs　is　developed　by　alkali-mediated　passivation　of　the　active　defect　states　(dangling　bonds)　on　CNNSs.　With　alkali　treatment,　the　active　defect　states　of　CNNSs　are　eliminated　and　replaced　with　the　passivated　-NH2　groups,　leading　to　significant　enhancement　of　the　FL　quantum　yields.　We　believe　that　this　work　will　promote　an　understanding　of　the　structure-FL　properties　of　CNNSs,　advancing　an　understanding　of　the　luminous　mechanisms　and　the　application　of　CNNSs　in　FL　sensing　as　well　as　biological　imaging.