As a light-intensity-controlling window, electrochromic layer has large potential applications. Thus it gains more and more attention. The properties of such electrochromic layers as WO3, NiOx, TiO2, V2O5, Ta2O5 have been studied in this paper. The influences of the evaporation factors, such as substrate temperature, vacuum, oxygen pressure and annealing, and their coloration mechanism, are discussed in detail. In this paper, we present the optical model of these layers, i.e. complex refractive index, N((lambda) ) equals n((lambda) )- ik((lambda) ), and the variations of N((lambda) ) with above conditions. We also studied the changes of N((lambda) ) when we apply positive and negative voltage to the layers. We find that the refractive index n((lambda) ) will become smaller and the extinction coefficient k((lambda) ) will become larger when electrochromic layer is colorized. And the curve of n((lambda) ) and k((lambda) ) will shift to longer wavelength. The longer wavelength absorption is due to impurity, defect, and lattice polarization.