A novel D-A alternative conjugated polymer PBDTDMCT containing benzo[1,2-b:4,5-b′]dithiophene (BDT) and dimethyl thiophene-3,4-dicarboxylate (DMCT), was designed and synthesized by Stille cross-coupling reaction. The copolymer exhibited excellent solubility and good thermal stability. The optical band gap determined from the onset of absorption of the polymer film was 2.10 eV. By incorporation of the ester groups into the polymer side chain, the HOMO level of polymer PBDTDMT was tuned to be deep-lying (−5.65 eV). Open-circuit voltage of polymer solar cells constructed based on PBDTDMT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) can be tuned to achieve values as high as ca. 1.0 V.