The global need for clean water and sanitation drives the development of eco-friendly and efficient water treatment technologies to combat biological pollution from pathogens. In this study, a novel heterojunction photocatalyst was synthesized by incorporating ZnIn₂S₄ into covalent organic frameworks (COFs) to enable environmentally friendly hydrogen peroxide (H₂O₂) photosynthesis and explore its potential for in situ disinfection. The ZnIn₂S₄/COF photocatalyst achieved remarkable H₂O₂ yields of 1325 µmol∙g⁻¹∙h⁻¹, surpassing pristine COF and ZnIn₂S₄ by factors of 3.12 and 16.2, respectively. The produced H₂O₂ was efficiently activated into hydroxyl radicals (·OH) through reaction with Fe(II), enabling rapid sterilization via a photocatalysis-self-Fenton system. Mechanistic insights, supported by physicochemical characterizations and theoretical calculations, highlighted the role of the internal electric field (IEF) in enhancing carrier separation and transfer, thereby boosting photosynthesis efficiency. This work presents a sustainable approach to H₂O₂ photosynthesis and activation for disinfection, offering a promising solution to global water treatment challenges.