To study the influence of the spacer between the donor (D) and acceptor (A) units in a side chain on the photophysical and photovoltaic performance, a novel side-chain D-A conjugated copolymer of PDPAT-BDT-BT is made, which contains binary donor units of N,N-diphenylthiophen-2-amine (DPAT) and benzo[1,2-b:4,5-b]dithiophene (BDT) in the main chain and an appending benzothiadiazole (BT) acceptor unit in the side chain. Compared with its counterpart of PDPAT-BDT-BT, which has an additional phenyl spacer between the DPAT and BT units, this PDPAT-BDT-BT exhibits a smaller optical bandgap, a broader absorption range, and a lower highest occupied molecular orbital (HOMO) energy level (-5.42 eV), as well as improved photovoltaic properties in bulk heterojunction polymer solar cells containing [6,6]-phenyl-C-71-butyric acid methyl ester as an electron acceptor. A maximum power conversion efficiency of 3.27% with an open circuit voltage of 0.87 V and a fill factor of 49.8% are obtained in the cells. This work indicates that the photophysical and photovoltaic performance of the side-chain D-A polymers in polymer solar cells can be significantly improved by shortening the phenyl spacer between the D and A units.