Abstract: A novel low band gap alternating copolymer, with 3,4-ethylenedioxythiophene (EDOT) as an electron donor and alkyl chains substituted tetraphenyl cyclopentadienone as an electron acceptor, was successfully synthesized via Stille cross-coupling polymerization. The new conjugated polymer was characterized by NMR, UV-Vis, GPC, CV, TGA and DSC. Results show that the macromolecule has good solubility and thermal stability with thermal decomposition temperature at 376.6 ℃. The number average molecular weight (Mn) is 1.45×104 with polydispersity of 1.13. UVVis spectroscopy indicates a low band gap (about 1.55 eV) of the copolymer and a broad absorption from 500 nm to 750 nm. A bulk heterojunction device with the new copolymer and ［6,6］-phenylC61 butyric acid methyl ester (PC60BM) as the activity layer was fabricated. It shows an open circuit voltage of 0.63 V, a short circuit current of 0.45 mA/cm2, the fill factor of 0.28 and the power conversion efficiency of 0.08%.