Defect　engineering　modified　graphite　carbon　nitride　(g-C3N4)　has　been　widely　used　in　various　photocatalytic　systems　due　to　the　enhanced　catalytic　activity　of　multiple　defect　sites　(such　as　vacancies　or　functional　groups).　However,　the　key　mechanism　of　action　in　each　defect　site　in　the　corresponding　photocatalytic　surface　reactions　is　still　unclear.　Here,　the　-C　N　groups　and　N　vacancies　were　sequentially　introduced　into　g-C3N4　(Nv-C　N-CN)　for　photocatalytic　production　of　high-value　and　multifunctional　H2O2,　and　the　effect　of　dual　defect　sites　on　the　overall　photocatalytic　conversion　process　was　systematically　analyzed.　The　modification　of　the　dual　defect　sites　forms　an　electron-rich　structure　and　leads　to　a　more　localized　charge　density　distribution,　which　not　only　enhances　the　light　absorption　and　carrier　separation　capabilities,　but　also　significantly　improves　the　selectivity　and　activity　of　H2O2　generation.　Importantly,　detailed　experimental　characterizations　and　theoretical　calculations　clearly　revealed　the　key　role　of　each　defect　site　in　the　photocataLytic　H2O2　surface　reaction　mechanism:　the　N　vacancies　can　effectively　adsorb　and　activate　O-2,　and　the　-C　N　groups　facilitate　the　adsorption　of　H+,　which　synergistically　promote　H2O2　generation.　The　Nv-C　N-CN　reached　a　H2O2　generation　rate　of　3093　mu　moL　g(-1)h(-1)　and　achieved　an　apparent　quantum　efficiency　of　36.2%　at　400　nm,　significantly　surpassing　the　previously　reported　g-C3N4-based　photocatalysts.　Meanwhile,　a　solar-to-chemical　conversion　efficiency　of　0.23%　was　achieved　in　pure　water.　Constructing　defects　and　understanding　their　crucial　role　provides　significant　insights　into　the　rational　use　of　defect　engineering　to　design　and　synthesize　highly　active　catalytic　materials　for　energy　conversion　and　environmental　remediation.