Graphitic　carbon　nitride　(g-C3N4,　CN)　is　widely　utilized　in　the　field　of　photocatalytic　hydrogen　production　owing　to　its　exceptional　visible　light　absorption,　addressing　the　challenge　of　energy　scarcity.　However,　its　development　has　been　hindered　by　the　issue　of　low　charge　separation　efficiency.　In　this　study,　an　ultrathin　(∼1.65　nm)　oxygen-doped　graphitic　carbon　nitride　nanosheets　loaded　amorphous　mesoporous　nickel　hydroxide　(a-Ni(OH)2/OCN)　photocatalyst　was　developed　through　a　straightforward　calcination　and　stirring　approach.　Under　visible　light　irradiation,　a-Ni(OH)2/OCN　exhibited　a　remarkably　high　photocatalytic　hydrogen　evolution　rate　of　4764.9　μmol·h−1　g−1,　which　is　4.7　times　greater　than　that　of　the　original　CN.　The　synergistic　effect　between　O-doping　and　heterojunction　was　proved　by　experimental　data　analysis　combined　with　DFT　calculations.　Totally,　the　introduction　of　oxygen　into　CN　led　to　a　reduction　in　its　band　gap,　while　a-Ni(OH)2/OCN　heterojunction　achieved　a　broaden　light　absorption　range　and　an　effective　charge　separation　due　to　a　type-Ⅱ　internal　electric　fields.　©　2023