Ammonium metatungstate, a low-cost industrial raw material, was taken as the precursor to prepare electrochromic WO3 films by a simple solution method.The microstructures of the WO3 films were characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy.The results show that WO3 films transform from amorphous phase into triclinic phase at about 400 ℃.The as-prepared films are uniform and show a very smooth morphology with a typical thickness of about 40 nm.The films exhibit a good electrochromism performance and the average transmittance modulation in range of 400 um to 1000 um increase with the annealing temperature, and the maximal value is about 26.1%.Hence, a simple method to prepare electrochromic WO3 films has been established and may be helpful in the industry product of electrochromic devices.