In recent years, polyselenophene and its derivatives have been paid more attention due to their unique properties such as lower band gaps and redox potentials. However, the poor solubility and molecular rigidity of polyselenophene and its derivatives limit them to use in flexible optoelectronic devices. Therefore, in order to enrich the structures and properties, fully understand the structure-property relationship and further improve the electrochromic performances of polyselenophene and its derivatives, we synthesized its propyl-analogues as precursors, to electrosynthesize three conducting polymers, poly(3,4-propylenedioxyselenophene) (PProDOS) and its alkyl-modified derivatives, poly(3,3'-dimethyl-3,4-dihydro-2H-selenopheno[3,4-b][1,4]dioxepine) (PProDOS-C-1) and poly(3,3'-dibutyl-3,4-dihydro-2H-selenopheno[3,4-b][1,4]dioxepine) (PProDOS-C-4). The electrochemical and spectroelectrochemical behaviors of PProDOS and PProDOS-C-n (n = 1, 4) were investigated in detail in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF(6)). Results showed that PProDOS-C-1 and PProDOS-C-4 exhibited superior electrochromic performances such as high optical contrast (62%) and excellent coloration efficiency (573 cm(2) C-1). In addition, as the alkyl chains increased, the onset oxidation potentials of monomers and the optical band gaps of polymer films decreased and the electrochemical activity of polymers increase. These polymers supply varied electrochromic colors and have prospect in display application.