A　porous　0D/2D　C3N4　nanocomposite　(PCN)　has　been　successfully　prepared　by　a　simple　and　green　one-pot　photochemical　oxidation　method　for　the　first　time.　Notably,　the　PCN　samples　are　easily　one-pot　synthesized　in　hundreds　of　grams　in　the　laboratory.　The　structure　features,　morphologies　and　photoabsorption　performances　of　the　samples　are　characterized　in　detail.　The　photocatalytic　activities　are　evaluated　by　hydrogen　evolution　under　visible　light　irradiation.　It　was　proven　that　the　green　chemical　scissor,　produced　by　the　photodecomposition　of　H2O2,　can　tailor　the　texture　of　layered　C3N4　to　form　porous　C3N4　nanosheets　and　C3N4　quantum　dots　(QDs)　without　doping　heteroatom　merely　through　the　destruction　of　part　of　triazine　rings,　and　the　as-prepared　PCN　sample　still　keeps　the　basic　chemical　structure　of　g-C3N4.　The　band　alignment　between　CN　QDs　and　C3N4　nanosheets　as　well　as　the　porous　structure　will　facilitate　the　separation　of　charge　carriers　and　provide　more　active　sites.　Therefore,　the　photocatalytic　hydrogen　evolution　rate　of　PCN　is　over　30　times　higher　than　that　of　the　parent　bulk　g-C3N4.　The　activity　is　also　higher　than　that　of　ultrathin　pore-free　C3N4　nanosheets　and　porous　C3N4　nanosheets.　Moreover,　the　photocatalytic　activity　of　PCN　could　be　further　improved　by　modifying　with　carbon　quantum　dots　(C　QDs).　Finally,　a　possible　photocatalytic　reaction　process　is　proposed.