A new optoelectronic building block, dithieno[3',2':3,4;2 '',3 '':5,6]benzo[1,2-c][1,2,5]thiadiazole, was designed by applying a fusion strategy on 4,7-dithienyl-2,1,3-benzothiadazole (DTBT) and named as fDTBT. In combination with benzo[1,2-b:4,5-b']dithiophene (BDT), fDTBT was used for the construction of a family of donor-acceptor copolymers, P(BDTn-fDTBT), with different side chains (n is carbon number of the side chain and varies from 8, 10, 12, 16, 20, to 24). It was found that the side chains have great impact on processing and photovoltaic properties of the polymers. P(BDTn-fDTBT) (n = 8, 10, and 12) bearing small alkyl side chains show poor solubility even in hot solvents. P(BDTn-fDTBT) (n = 20 and 24) have good solubility but inferior photovoltaic performance with an efficiency of 1.04% and 0.49%, respectively. Only P(BDT16-fDTBT) having 2-hexyldecyl side chain possesses both suitable solution processability and good photovoltaic properties with an efficiency around 4.36%. The comparison between P(BDT16-fDTBT) with the nonfused reference polymer P(BDT20-DTBT) reveals that the structural fusion on DTBT endows the polymer a deeper HOMO energy level and a better film morphology when blending with [6,6]-phenyl-C-61-butyric acid methyl ester (PC61BM), finally resulting in improved photovoltaic performance.