In order to improve the electrochemical cycling stability of electrochromic tungsten oxide (WO3) thin films, Ta-doping WO3 films are prepared by magnetron sputtering method. Microstructure, optical and electrochromic properties of the films are investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), spectra ellipsmetry (SE), ultraviolet-visible spectrophotometer and cyclic voltammetry, respectively. The influence of Ta-doping on the structure of WO3 thin film and electro-chromism is studied. The results indicate that incorporation of proper content of Ta in WO3 can control the microstructure of the film, which decreases the crack and makes the film smooth. However, excessive Ta can make film dense, or even form agglomerates, which can adversely affect ion transport in the films. Compared to pure WO3 film, the optical modulation, color efficiency and cyclic stability of the Ta-doping WO3 film are improved.