Z-scheme heterojunction photocatalysts (ZsHP) have attracted significant attention due to their excellent carrier recombination suppression and charge potential enhancement properties. However, the photocatalytic performance of ZsHP is limited by inefficient charge transfer at the heterojunction interface, mainly due to electron trapping by O-2 or H+ in part B and the large energy gap between its conduction band (CB) and the valence band (VB) of part A. Herein, perylene tetracarboxylic diimide (PTCDI, part B) and g-C3N4 (part A) are used to fabricate an organic ZsHP (PI-g-C3N4) as a novel strategy to significantly improve interfacial charge transfer efficiency by tailoring the Br substitution at the PTCDI bay position. Br substitution at the bay position reduce electron trapping in part B and bring the CB of part B closer to the VB of part A. Moreover, Br substitution enhance the electric field from g-C3N4 to PTCDI, enhancing the interfacial charge transfer of the ZsHP. These findings enable the design of ZsHP with tunable interface electron-transfer efficiencies.