Amorphous WO3 electrochromic films are deposited by advanced mid-frequency dual-target magnetron sputtering method using pure tungsten as targets. X-ray diffraction (XRD), X-ray photoelectronic spectroscopy (XPS) and ultra violet spectrophotometer are used to analyze the structure, morphology, composition and transmittance property of the films respectively. The effects of oxygen flow and heat treatment temperature on the electrochromism of the films are studied. The results show this method is available to deposit electrochromic films. The as-deposited films deposited at room temperature are amorphous. It can improve the electrochromic performance of films to raise oxygen content and keep appropriate heat treatment temperature. In the experiment, films deposited at high oxygen content show favorable electrochromic property after annealing at 200 ℃, and the variation of average transmittance difference between the bleached and colored state reaches 50% in the wavelength range 380～780 nm.