Polymer heterojunctions (PHJs) have emerged as promising photocatalysts for the photocatalytic hydrogen evolution (PHE). Nevertheless, most PHJs exhibit unsatisfactory hydrogen evolution rate (HER), primarily attributing to their own high‐energy Frenkel excitons and poor light capturing ability. In this paper, a molecular engineering strategy is developed to further broaden spectral response range and simultaneously accelerate Frenkel excitons dissociation within PHJs. For this purpose, three donor–acceptor (D‐A) conjugated polymers/g‐C3N4 heterojunctions with alternative donor units (fluorene, carbazole, N‐annulated perylene for P1, P2, and P3, respectively) and the invariant acceptor unit (benzothiadiazole) have been designed and fabricated for efficient PHE. Experimental results show that copolymerizing different donor units into the polymer skeleton not only extends the visible‐light response range but also promotes photoexciton separation within polymer/g‐C3N4 PHJs. Notably, copolymerizing the strongest electron donor unit (N‐annulated perylene) achieves the best light capture ability and the most effective photoexcitation separation of the P3/g‐C3N4, leading to significantly increase HRE of 13.0 mmol h−1 g−1 with a recorded apparent quantum yield of 27.32% at 520 nm. Importantly, the Type II heterojunction mechanism within P3/CN was first proved by theoretical calculation. This work provides a promising strategy for reasonably developing efficient PHJs for solar fuel production. A molecular engineering strategy based on introducing donor–acceptor units into conjugated polymers for polymer heterojunction photocatalysts with broadened spectral response range and simultaneously accelerate Frenkel excitons dissociation is developed. Notably, N‐annulated perylene polymers achieve a AQY of 27.32% at 520 nm.