Abstract: The principle of electrochromatic system involving dieledric layer is studied.The preparation parameters of G/ITO/WO3/Ta2O5/ITO system produced by electron beam evaporation are showed. the effect of adsorbed water contained in dielectric film on coloration is also analyzed. In addition, the other factors that have influence on the system performance, such as processing parameters, the sheet resistance of ITO film and the strength of applied voltage, are discussed. Finally, the requirements for dielectric layer are presented.