Poly(heptazine　imide)　(PHI)　is　a　promising　photocatalyst　for　hydrogen　peroxide　(H2O2)　production;　however,　enhancing　its　specific　surface　area　to　expose　internal　active　sites　and　understanding　their　roles　in　key　mechanistic　steps　for　the　H2O2　synthesis　remain　challenging.　Here,　we　utilized　organic　cations　to　exfoliate　bulk　PHI　and　fabricate　PHI　nanosheets　for　producing　H2O2　at　a　rate　of　27.35 mmol　g−1　h−1　under　simulated　solar　light　irradiation,　outperforming　most　of　the　reported　carbon　nitride-based　catalysts.　Importantly,　after　36 h　of　cyclic　accumulation　reactions　in　a　self-created　spiral　flow　reactor,　the　H2O2　concentration　stabilized　at　2.7 wt.%,　close　to　medical　sterilization　levels.　In　situ　spectroscopic　characterizations　and　density　functional　theory　calculations　revealed　that　the　exfoliation　results　in　molecular　reconfiguration　of　the　PHI　basal　planes,　forming　the　active　sites　to　promote　charge　separation　and　electron　localization.　This　new　structure　also　creates　midgap　states,　enabling　direct　H2O2　production　via　a　one-step,　two-electron　pathway,　bypassing　the　superoxide　radical　pathway.　Theoretical　calculations　suggest　that　the　localized　electronic　structure　created　by　the　active　sites　favors　the　protonation　of　adsorbed　O2　and　stabilizes　the　*OOH　species,　which　converts　to　H2O2.　This　study　elucidates　and　underscores　the　importance　of　active-site　reconfiguration　for　efficient　photocatalytic　oxygen　reduction　reaction　(ORR)　pathways.　©　2025　Wiley-VCH　GmbH.