In this work, three new nonfullerene acceptors (BT6IC-BO-4Cl, BT6IC-HD-4Cl, and BT6IC-OD-4Cl), which comprise a central fused benzothiadiazole core and two dichlorinated end groups and substituted with different branched alkyl chains [2-butyloctyl (BO), longer 2-hexyldecyl (HD), and 2-octyldodecyl (OD)], are successfully designed and prepared. The influences of the branched alkyl chain with different lengths on the electronic/optoelectronic property, electrochemistry, and photovoltaic performance are systematically investigated. It has been revealed that BT6IC-HD-4Cl, which had the medium alkyl chain (2-hexyldecyl) length, has the best photovoltaic performance when using PDBT-TF as the electron donor. The BT6IC-HD-4Cl-based device shows an impressive power conversion efficiency of 14.90%, much higher than BT6IC-BO-4Cl (14.45%)- and BT6IC-OD-4Cl (9.60%)-based devices. All these evidence shows that the subtle changes in the alkyl substituent of these high-performance chlorinated acceptors can have a big impact on the structural order and molecular packing of the resultant nonfullerene acceptors and ultimately on the photovoltaic performance of the final solar devices.