Since recent appearance of “Y-series” non-fullerene acceptors (NFAs), the field of organic solar cells (OSCs) began to show high power conversion efficiencies (PCEs). Side chain engineering of NFAs is an important strategy to improve PCEs. Herein, we designed and synthesized two NFAs with (BTA-UD-4F) and without (BTA-4F) undecyl chain on thienothiophene moiety based on the n-hexyl side chain on benzotriazole moiety. In addition, we studied the impact of different side chains on thienothiophene moiety. And we fully investigated their thermal, optical, electrochemical properties and device performances. Our studies showed that two NFAs exhibit good electron mobility and surface morphology, but BTA-UD-4F with undecyl chain has better electron mobility and smoother morphology compared to BTA-4F. Therefore, PM7:BTA-UD-4F based on NFOSCs showed better short-circuit current density (Jsc). As a result, PM7:BTA-UD-4F achieved high PCE of 9.12% with an open-circuit voltage (Voc) of 0.71 V, a Jsc of 21.08 mA/cm2 and a fill factor (FF) of 62.04%. This studies demonstrate that introduction of side chain in appropriate position can improve PCE of OSCs.