A direct synthesis strategy was employed to prepare functionalized COFs enriched with acidic sites, using various precursor monomers. The functionalized COFs were applied in the catalytic conversion of biomass-derived xylan to furfural in the liquid phase. The study further assessed the recyclability and reusability of these COFs, explored the relationship between their structural features and catalytic performance, and investigated the reaction mechanism underlying the COF-catalyzed conversion of xylan to furfural. The results revealed the successful solvothermal synthesis of four COFs (COF-LZU1, TP-PA, TB-DAB, and TP-DAB). Among them, TP-DAB exhibited the highest acidity, reaching 2.092 mmol/g, due to its rich content of -OH and -COOH. Additionally, TP-DAB demonstrated a well-developed porous structure and excellent thermal stability, essential characteristics for a highly efficient solid acid catalyst. When employed as a catalyst in a mixed solvent system of H