Dimerized small-molecule acceptors (SMAs) built by the conventional connection of terminal groups of monomers have contributed to exciting long-term stabilities of organic solar cells (OSCs). However, device efficiencies, especially fill factors (FFs), still need to be improved. This probably originates from unsymmetrical molecular structure/conformation-determined less compact/ordered molecular stackings, such as ineffective stackings of constraint terminals. Herein, an exotic dimerized SMA of BC-Th is established by bridging the branched groups (BC-type, branch coupling) of two monomers rather than conventional terminal units (TC-type, terminal coupling). Benefiting from the three-dimensional conformation and more uncurbed terminals, BC-Th exhibits multiple molecular orientations along with a larger dielectric constant and electron mobility compared with TC-Th. Finally, an efficiency of 17.43% is achieved by BC-Th-based OSCs, along with the highest FF of 79.13% among all dimerized SMAs-based OSCs to date. When introducing L8-BO as the third component, overall enhanced efficiency of 18.05% and FF of 80.11% are further afforded. Contrarily, TC-Th-based OSCs exhibit much inferior PCE of 16.29% and FF of 74.81%, demonstrating the great advantages of "branch coupling" over "terminal coupling" when building dimerized SMAs.