Two D‐A copolymers consisting of fused ring pyrrolo‐dithieno‐quinoxaline acceptors are synthesized with different donor units, i.e., benzodithiophene (BDT) with alkylthienyl (P134) and 2‐ethylhexyloxy (P117) side chains. These copolymers are used as donors and a narrow bandgap acceptor Y6 to fabricate bulk heterojunction polymer solar cell devices. Owing to the strong electron‐deficient fused ring pyrrolo‐bithieno‐quinoxaline and weak alkyl thienyl side chains in BDT, the polymer solar cells (PSCs) based on P134:Y6 attain the power conversion efficiency (PCE) of 15.42%, which is higher than the P117:Y6 counterpart (12.14%). The superior value of PCE for P134:Y6 can be associated with more well‐adjusted charge transport, weak charge recombination, proficient exciton generation, and dissociation into free charge carriers and their subsequent charge collection owing to the dense π–π stacking distance and more considerable crystal coherence length for the P134:Y6 thin films. This investigation confirms the great potential of a strong acceptor‐weak donor tactic for developing efficient D‐A copolymers consists of quinoxaline acceptor for PSCs.