The photocatalytic oxygen reduction reaction (ORR) towards hydrogen peroxide (H₂O₂) is a promising but challenging alternative to the industrial anthraquinone process. Crystalline porous covalent organic frameworks (COFs), a new generation of semiconductors, have attracted significant attention because they exhibit broad visible-light harvesting, possess tunable energy levels, and facilitate facile separation and migration of photogenerated charge pairs. The modulation of the energy levels of COFs could facilitate charge transport, resulting in better photocatalytic performance. Herein, the π-conjugation system of COFs was extended via a post-sulfurization process, leading to a considerably enhanced photocatalytic activity. During the sulfurization process, imine-linked COFs were converted into thiazole-linked COFs to extend the conjugation structure of COFs in the x and y directions, resulting in higher transport of electrons. The thiazole-linked 4PE-N-S presents the best photocatalytic H₂O₂ generation performance with a rate of 1574 μmol g⁻¹ h⁻¹, which is about 5.8 and 3.7 times higher than that of imine-linked 4PE-N and 4PE-TT, respectively. Our findings provide new prospects for the design and synthesis of highly active organic photocatalysts for solar-to-chemical energy conversion.