(E)-5-(2-(5-(Alkylthio)thiophen-2-yl)vinyl)thiophene-2-yl functional groups were introduced onto 4- and 8-positions of BDT units, and this building block was used to construct a new derivative polymer of PTB7, named as PBT-TVT. Benefiting from the prolonged conjugation of the conjugated side groups on BDT units, the optical absorption property of PBT-TVT can be improved greatly compared to that of PTB7, so an inspiring result of 7.67% was obtained by using PBT-TVT as the donor and PC61BM as the acceptor in polymer solar cells (PSCs), which is much higher than that of the PTB7:PC61BM-based device and also one of the highest results for PSCs with PC61BM. In electrochemical cyclic voltammetry (CV) measurements, PBT-TVT showed a deeper HOMO level than PTB7 so the device based on the former exhibits higher open circuit voltage than the latter. Moreover, in comparison with PTB7, the new polymer PBT-TVT exhibited stronger interchain pi-pi interaction and thus higher hole mobility. Overall, the results in this work indicated that PBT-TVT is a promising donor polymer, and the strategy used in this work will be beneficial for molecular design of polymer photovoltaic materials for large-scale production of PSCs.