Abstract: Nanocrystalline WO₃ thin films is prepared by RF reactive sputtering in this paper.Based on the analysis of the Berg hysteresis model,we analyze the process parameter influence on the hysteresis effect,inclusing pumping speed,temperature of substrate and target,input power,target to substrate distance and area of target.A new active feedback control method is present in this paper to eliminate hysteresis effect and depositing nanocrystalline WO₃ thin films.Through measurement of its electro-discoloring properties,we get a good tungsten oxide films.The tungsten oxide films have a high visible light transmission rate before discoloring,the maximum value reach 95% at the visible light wave length 550nm.After discoloring,visible light transmission rate decrease to 30% at wave length 550nm.XPS and XRD results show that the tungsten oxide films is nanocrystalline.The nanocrystalline WO₃ has a very high ratio surface area and those surface defect provide ion more diffusion channel,so nano-crystaln has a good electro-discoloring property.The better size of Nano-crystal is about 40nm.