Photocatalytic　water　oxidation　half-reaction　is　a　critical　bottleneck　in　achieving　efficient　overall　water　splitting　using　polymeric　photocatalysts,　primarily　due　to　the　lack　of　efficient　water　oxidation　sites　and　sluggish　reaction　kinetics.　To　address　these　challenges,　a　doping　engineering　strategy　is　proposed　to　activate　inert　carbon　atoms　as　efficient　water　oxidation　sites　in　poly(heptazine　imide)　(PHI).　Computational　analysis　revealed　that　Rb+　dopants　can　induce　in-plane　structural　distortions　in　PHI,　thereby　modulating　the　electronic　structure　of　carbon　atoms.　This　modulation　enhanced　the　adsorption　of　*OH　intermediates　on　carbon　atoms　and　significantly　reduced　the　reaction　barrier　for　water　oxidation.　Furthermore,　charge　carriers　dynamics　uncovered　that　the　incorporation　of　the　Rb+　ions　into　the　PHI　framework　can　promote　the　photoexcited　exciton　dissociation　and　induce　the　holes　to　migrate　to　the　water　oxidation　sites.　As　a　result,　the　Rb-doped　PHI　achieved　an　excellent　apparent　quantum　efficiency　value　of　up　to　20.4%　at　400 nm　for　water　oxidation.　This　finding　offers　a　new　channel　for　researchers　to　develop　highly　efficient　polymeric　photocatalysts　for　photocatalytic　water　oxidation　reactions.　©　2025　Wiley-VCH　GmbH.