Chlorine-substituted conjugated polymers not only possess simple and high-yield synthetic steps due to the cheap raw materials available for their synthesis but also exhibit great potential for the commercial application of polymer solar cells (PSCs) with excellent photovoltaic performance. However, the current research of chlorine-substituted donor polymers is lagging compared to the rapid development of chlorinated acceptors. Here, a chlorine-free polymer PTBT and a chlorinated donor polymer PTBT-Cl were prepared and studied in parallel. The chlorine-substituted polymer PTBT-Cl showed a much lower highest occupied molecular orbital (HOMO) level value of 5.46 eV and an enhanced aggregation behavior in film, achieving an optimized phase separation morphology in blend with an efficient acceptor compared with that of PTBT. Surprisingly, the chlorine-free PTBT-based PSCs showed a negligible power conversion efficiency (PCE) of 3.88%, while the PTBT-Cl exhibited a decent PCE of 12.31%, with a simultaneously increased short circuit current density (J(sc)) of 23.73 mA cm(-2) and a fill factor (FF) of 62.53%, when matched with the fused-ring electron acceptor (FREA) Y6. This work provides new insight into the economical design and synthesis of chlorine-substituted polymers, boosting the photovoltaic performance of PSCs.