Chlorination has proved to be an efficient method in tuning energy levels and optoelectronic properties of organic semiconducting materials with cheaper synthesis and low-cost raw materials when compared with the fluorinated analogues. Herein, in this work, 5-chlorobenzo[c][1,2,5]thiadizole (Cl-BT) unit was chosen as a new acceptor unit to design two new D-A type electrochromic conjugated polymer: P(Cl-T) and P(Cl-EDOT). The electrochemical behavior and electrochromic performances of above-mentioned chlorinated polymers were studied by electrochemical polymerization techniques and spectroelectrochemistry. Thanks to the strong electron-withdrawing ability of Cl-BT unit and the strong electron-donating properties of EDOT unit, the resultant Cl-EDOT precursor could electropolymerize into high quality hybrid polymer films at lower potential (0.71 V vs. Ag/AgCl) with favorable redox activity and better redox stability. Spectroelectrochemistry shown that as-formed lower band gaps P(Cl-EDOT) exhibits reversible color-changing nature from pale blue to gray accompanied with the moderate optical contrast (13.9%), fast response time (0.6 s), and favorable coloration efficiency (126.4 cm2 C−1) in the NIR region that was measured from the kinetic studies. By optimizing the tested electrolyte and solution, its electrochromic performance is further improved (optical contrast: 25.6%, response time: 0.25 s, coloration efficiency: 275.9 C−1 cm2). The favorable redox stability and electrochromic performance of P(Cl-EDOT) make it a good candidate to construct electrochromic device, and Cl-BT unit should be a promising building block in designing high performance near infrared electrochromic materials. [Display omitted] • Two electrochromic precursor based on 5-chlorobenzo[c][1,2,5]thiadizole unit were synthesized by Stille coupling reaction. • Different donor brings different influence on the photophysical properties of the chlorinated precursors and its polymers. • The introduction of EDOT unit is beneficial for the preparation of better-quality chlorinated polymers. • P(Cl-EDOT) exhibit better redox activity and stability, and satisfied electrochromic performances. • Chlorinated benzothiadiazole could be a promising building blocks in constructing high performance D-A conjugated polymers. [ABSTRACT FROM AUTHOR]