A novel series of comonomers, which comprise dibenzothiophene (DBT) and dibenzofuran (DBF) cores symmetrically linked to thiophene and 3-alkylthiophenes at 2 and 8-positions, were designed and electropolymerized to yield the corresponding electrochromic polymers. The structure–property relationships of comonomers and polymers, including electrochemistry, thermal stability, fluorescence, and electrochromic properties, were systematically explored. In relation to the core group, the alkyl chain group of these polymers had a relatively significant influence on the redox behavior, band gap, neutral state colour, stability, and electrochromic performance (optical contrast, CE, and switching time) of the system. Furthermore, all the polymer films displayed unique electrochromic characteristics with switching the color from yellow to blue. Further kinetic results showed moderate to high optical contrast (20–70%), high colouration efficiency (typically 170–370 cm² C⁻¹), and favorable switching time (0.8–9.4 s). Among them, the electrochromic performances of 3-hexylthiophene-end-capped polymers were superior to those with thiophene/3-methylthiophene as terminal groups. These results demonstrated that DBT/DBF-based π-conjugated polymer materials hold promise for display applications and DBT/DBF could be further employed for the rational design of excellent electrochromic polymers by matching with other heterocycle units.