In this work, a thin film of poly(3,4-ethylenedioxythiophene) (PEDOT) was modified with chloride anions (Cl^-), intending to enhance its electrochromic (EC) performance. The modified film of PEDOT was prepared by electrochemical polymerization in an aqueous solution, containing the monomer and Cl^-. The resultant film is abbreviated as PEDOT-Cl. The PEDOT-Cl film prepared in the aqueous solution exhibited multi-color EC properties, with reference to those of bare PEDOT film prepared in the organic solvent, acetonitrile. The PEDOT-Cl film showed two electrochromic states in the visible region, upon the application of voltage from 0.3 ~ -0.8 V (vs. Ag/Ag⁺); the modified film exhibited light blue and red brown at the oxidized state and reduced state, respectively. Normally, the pristine PEDOT thin film can be switched from deep blue to light blue; however, the phenomenon we observed here is dramatically different from what have been reported in the literatures, presumably due to the doped cloride ions inside the PEDOT film. The enhanced electrochromic properties of the thin film of PEDOT-Cl have apparently something to do with the incorporation of Cl^-. The transient transmittance of the PEDOT-Cl film gave a transmittance window (ΔT) of 31% at 500 nm. For an active area of 2.0 x 2.0 cm², the switching times of the PEDOT-Cl film were estimated to be 4.4 and 4.0 s for bleaching and coloring, respectively. The coloration efficiency of the PEDOT-Cl film was calculated to be 215.3 cm²C^-1 at 500 nm.