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.
