Covalent organic frameworks (COFs) show great promise for photocatalytic hydrogen production due to their tunable electronic structures and functionalities. However, the high exciton binding energy and insufficient charge separation capabilities limit the COFs' photocatalytic efficiency. Herein, we performed molecular-level functionalization by introducing hydroxyl or methoxy groups into the COF, and explored in depth the impact of hydroxyl functionalization on the photogenerated carrier behavior in the COF. Through detailed experimental and theoretical studies, we reveal that the introduction of hydroxyl groups could enhance charge separation and stabilization, effectively suppressing the recombination of photo-generated electron-hole pairs, and thereby reducing exciton binding energy. As a result, the HITMS-COF-3 with fully hydroxyl substitutes achieves a hydrogen production rate over 65,310 & micro;mol g-1 h-1 with a quantum yield of 6.9% at 520 nm, which is a 3.7-fold increase compared to those obtained with fully mehoxyl substituted COF (HITMS-COF-1). HITMS-COF-3 also achieves a higher electron transfer number (0.46 e-) and a prolonged excited state lifetime (approximate to 2 ns). The results indicate that modulating the COFs chemical microenvironment provides valuable insights into solar fuels with efficient organic photocatalysts.