<p>Conjugated covalent organic frameworks (COFs) possess distinct advantages over amorphous organic photocatalysts, including high charge transfer efficiency and effective light utilization. However, their photocatalytic performance often declines under prolonged solar irradiation in air, largely owing to their limited photostability. In this study, we systematically investigated the photobleaching mechanism of vinylene-linked COFs synthesized <i>via</i> aldol condensation between 2,4,6-trimethyl-1,3,5-triazine and aldehydes. When the COF contains phenylene as the linker, oxygen can undergo photoinduced addition across the vinylene bond, leading to cleavage of the vinylene bonds and the formation of terminal aldehydes, thereby causing photobleaching. Mechanistic studies indicated that superoxide anion radicals play a key role in this oxidative degradation process, accompanied by the partial cycloaddition of adjacent vinylene bonds. To enhance photostability, we incorporated naphthyl groups into the framework at the strut, replacing phenylene to modulate the excited-state electronic structure at the vinylene linkages. The newly synthesized COF exhibited notable anti-photobleaching capability in the presence of oxygen. This work not only deepens the understanding of photodegradation mechanisms in conjugated COFs but also offers valuable insights for the design and application of photostable COF-based catalysts.</p>