The construction of donor-acceptor (D-A) molecular structure is an attractive strategy to enhance the photocatalytic performance of polymeric semiconductors. Herein, dibenzothiophene (DBT)-4-carbaldehyde as the precursor is introduced into g-C₃N₄ (TCN) prepared by two-step thermal polymerization to construct an intramolecular D-A type copolymer (TCN-DBT_x). DFT calculation and experimental results reveal that DBT plays the role of electron donor unit to modify g-C₃N₄. The incorporation of DBT not only adjusts the band gap to improve reduction ability, but also induces an internal electric field with extending π-conjugated system for effective charge transfer. As a result, TCN-DBT_x exhibits much better photocatalytic performance with an optimal hydrogen production rate of 3334 μmol h^-1 g^-1, which is 2.5 times that of TCN. This work provides a protocol for preparing high-performance g-C₃N₄-based photocatalysts toward various applications.