Nanocrystalline tungsten oxide thin films were prepared by colloid chemistry method. By varying pH in the solution from which the films were prepared, the corresponding structure, constituent phase and photochromic behavior of the films were investigated with transmission electron microscope (TEM), X-ray diffraction (XRD), absorption spectra and X-ray photoelectron spectra (XPS). It was found that with the increase of the pH of the solution, tungsten oxide hydrate was gradually transformed into an oxided 12-tungstate with Keggin structure. XPS detection revealed that oxided 12-tungstate has a different reduction state from tungsten oxide hydrate when exposed to ultraviolet light, so as to result in the change of photochromic behavior of the films. And the variation of reduction characteristics of the films is believed to be related to the structural transformation of the films caused by pH in the solution.