A medium-band-gap donor-acceptor (D-A) polymer (PBDTS-DTBO) was designed and synthesized with alkoxyl-substituted benzo[c][1,2,5]oxadiazole (BO) as A unit and benzo [1,2-b:4,5-b]dithiophene (BDT) derivative (BDTTS) as D moiety. The resulting polymer not only possesses a high number-average molecular weight (M-n) of 53.0 kDa, but also exhibits a deep highest occupied molecular orbital (HOMO) energy level of - 5.43 eV, which is in favor of a higher V-oc of 0.88 V in the photovoltaic devices. Consequently, due to excellent phase separation and weak bimolecular recombination, the efficiency of the fullerene-based solar cells can reach 8.47% with a high current density (J(SC)) of 14.93 mA cm(-2) and FF of 0.661, which is one of highest values for currently reported benzoxadiazole-based materials. In addition, considering the polymer also shows a medium optical bandgap of 1.75 eV, which can be well complementary with the classical non-fullerene acceptor ITIC. The best performance device exhibits a good efficiency of 6.45% in fullerene-free devices, which could be ascribed to the serious bimolecular recombination. This work revealed that alkoxyl-substituted benzoxadiazole could be a promising moiety to construct efficient light-harvesting polymer.