High-quality poly(5-formylindole) (P5FIn) films with conductivity of 2.43 × 10⁻² S cm⁻¹ were synthesized electrochemically by direct anodic oxidation of 5-formylindole (5FIn). The electrochemical, thermal, fluorescent and spectroelectrochemical characterizations of this novel polyindole derivative were reported. Fluorescent spectral studies indicate that the P5FIn film with high fluorescence quantum yields and photochemical stability is a novel green-light-emitter. Spectroelectrochemical studies show that P5FIn can change from yellow to green along with the increasing potentials. Furthermore, a dual-type electrochromic device (ECD) based on P5FIn and poly(3,4-ethylenedioxythiophene) (PEDOT) was constructed and its electrochromic properties were investigated. The response time of this device was found to be only 0.9 s and the optical contrast was 56%. The coloration efficiency (CE) was calculated to be 582 cm²C⁻¹. The ECD exhibits a color change from green (neutral) to dark blue (oxidized) with a good open circuit memory and robust cycle life.