Conjugated　polymers,　represented　by　polymeric　carbon　nitrides　(PCNs),　have　risen　to　prominence　as　new-generation　photocatalysts　for　overall　water　splitting　(OWS).　Despite　considerable　efforts,　achieving　highly　crystalline　PCNs　with　minimal　structural　defects　remains　a　great　challenge,　and　it　is　also　difficult　to　examine　the　exact　impact　of　complex　defect　states　on　OWS　process,　which　largely　limits　their　quantum　efficiency.　Herein,　we　devise　a　‘in-situ　salt　flux’　assisted　copolymerization　protocol　by　using　nitrogen-rich　and　nitrogen-deficient　monomers　to　precisely　manipulate　the　structural　defects　of　poly　(triazine　imide)　(PTI)　single　crystals.　Stoichiometric　control　between　two　comonomers　enables　continuous　tunning　of　carbon-　and　nitrogen-vacancies　within　PTI,　allowing　the　construction　of　a　series　of　PTI　crystals　with　different　defect　states.　Theoretical　and　experimental　results　unveil　the　carbon　vacancies　are　related　with　the　radiative　decay　of　excitons,　while　the　nonradiative　decay　is　mainly　derived　from　the　nitrogen　vacancies.　Owing　to　the　effective　suppression　of　both　radiative　and　nonradiative　losses,　the　as-synthesized　PTI　achieves　a　record　apparent　quantum　efficiency　of　37.8　%　by　one-step-excitation　OWS.　This　work　highlights　the　significance　of　rational　control　of　the　structural　defects　and　describes　clear　structure-property-activity　relationships　in　PTI　photocatalyst,　offering　guidance　for　the　development　of　polymer　photocatalysts　for　solar　fuel　production.　©　2025　Wiley-VCH　GmbH.