Morphology is crucial to determining the photovoltaic performance of organic solar cells (OSCs). However, manipulating morphology involving only small-molecule donors and acceptors is extremely challenging. Herein, a simple terminal alkyl chain engineering process is introduced to fine-tune the morphology towards high-performance all-small-molecule (ASM) OSCs. We successfully chose a chlorinated two-dimension benzo[1,2-b:4,5-b′]dithiophene (BDT) central unit and two isomeric alkyl cyanoacetate as the end-capped moieties to conveniently synthesize two isomeric small-molecule donors, namely, BT-RO-Cl and BT-REH-Cl, each bearing linear n-octyl (O) as the terminal alkyl chain and another branched 2-ethylhexyl (EH) as the terminal alkyl chain. The terminal alkyl chain engineering process provided BT-RO-Cl with 13.35% efficiency and BT-REH-Cl with 13.90% efficiency ASM OSCs, both with Y6 as the electron acceptor. The successful performance resulted from uniform phase separation and the favorable combination of face-on and edge-on molecular stacking of blended small-molecule donors and acceptors, which formed a fluent 3D transport channel and thus delivered high and balanced carrier mobilities. These findings demonstrate that alkyl chain engineering can finely control the morphology of ASM OSCs, and provides an alternative for the optimal design of small-molecule materials towards high-performance ASM OSCs.