Side chains generally dictate molecular packing and film morphology and critically affect the efficiency of organic solar cells (OSCs), and hence, side-chain engineering plays a substantial role in achieving high-performance OSCs. In this work, a series of non-fullerene acceptor molecules with A–D–A structures, L6–L11, having gradient branched alkyl-chains on dithieno[3,2-b:2′,3′-d]pyrrole (DTP)-based asymmetrical chlorinated acceptors were designed and synthesized. The effects of branched alkyl-chain length, ranging from n-butyl to 2-decyldodecyl chains, on their optoelectronic properties, thin film molecular packing, blend film morphology and overall photovoltaic performance were systematically studied. Interestingly, the results indicated that with the increase in alkyl-chain length, the open-circuit-voltage (V_OC) is monotonously increased, while the short-circuit current density (J_SC), fill factor (FF) and power conversion efficiencies (PCEs) perceive a distinct parabolic trend. The reasons for the variation trend of photoelectric parameters were analyzed. Finally, a 2-butyloctyl chain-containing acceptor L8-based device demonstrated a champion PCE of 15.40% with a V_OC of 0.864 V, a J_SC of 23.63 mA cm⁻² and an FF of 0.754, which is the highest PCE for non-fullerene binary OSCs based on asymmetric ITIC-type acceptors. Further studies indicate that the proper 2-butyloctyl side chain could induce more favorable face-on molecule orientation, enhance carrier mobility, balance charge transport and suppress recombination loss. Our results will provide valuable guidelines for accelerating the understanding of the acceptor structure-photovoltaic performance relationship of OSC materials.