Two indenothiophene-based non-fullerene small molecule acceptors (NFSMAs), FTBT and FTTBT, were designed and synthesized to investigate the photovoltaic effect of fusing a thiophene into the core of NFSMAs. Compared with the none-fused FTBT, the thiophene-fused FTTBT achieves a much higher power conversion efficiency (PCE) of 9.79% with a Voc of 0.934 V, a Jsc of 16.01 mA cm-2 and an FF of 65.49%, when it was blended with PM6 polymer donor to fabricate bulk-heterojunction solar cells. Combined photophysical, electrochemical, photovoltaic property and morphology analysis indicates that the boosted device performance mainly lies in two reasons: (i) the incorporation of an electron-donating thiophene ring narrows the optical bandgap by extending π-conjugation, which contributes to a large short-circuit current; (ii) the incorporation of a single thiophene ring transforms the axisymmetrical molecular configuration into a centrosymmetrical one, which decreases the crystallinity and optimizes the packing feature in the blend. It results in a transport-favorable blending morphology and contributes to a high fill factor. The work clarifies an effective molecular design strategy for performance enhancement of organic solar cells.