Organic solar cells (OSCs) have advanced rapidly due to the development of new photovoltaic materials. However, the long-term stability of OSCs still poses a severe challenge for their commercial deployment. To address this issue, a dimer acceptor (dT9TBO) with flexible linker is developed for incorporation into small-molecule acceptors to form molecular alloy with enhanced intermolecular packing and suppressed molecular diffusion to stabilize active layer morphology. Consequently, the PM6 : Y6 : dT9TBO-based device displays an improved power conversion efficiency (PCE) of 18.41 % with excellent thermal stability and negligible decay after being aged at 65 degrees C for 1800 h. Moreover, the PM6 : Y6 : dT9TBO-based flexible OSC also exhibits excellent mechanical durability, maintaining 95 % of its initial PCE after being bended repetitively for 1500 cycles. This work provides a simple and effective way to fine-tune the molecular packing with stabilized morphology to overcome the trade-off between OSC efficiency and stability.