A novel reversible photochromic nanocomposite film based on a hybrid inorganic-organic matrix in which het-eropolyacid H4SiW12O40 (SiWA) was entrapped in Si--O-Ti network was prepared. Structure, photochromic behaviors and mechanism of the film were investigated by means of infrared spectroscopy (IR), ultraviolet-visible (UV-Vis) absorption spectra and electron spin resonance (ESR). The results showed that heteropolyanion I.e. SiW12O404- (SiW12), maintained a Keggin structure in the film and there was a strong interaction between anion SiW12 and cation R-NH+ (R=link of hybrid compos-ite). The photochromic properties of the composite film originated from reversible charge transfer between the anions and cations. Under UV irradiation, the anion would be reduced via one-electron step with simultaneous oxidation of the cation, accompanied by a color change from colorless to blue. Bleaching could occur when the film was in contact with ambient air or O2 in the dark.