A novel reversible photochromic nanocomposite film based on a hybrid inorganic-organic matrix in which heteropolyacid H3PW12O40, (PWA) was entrapped was prepared. The structure, photochromic behaviors and mechanism of the film were investigated by means of infrared (IR) spectroscopy, UV-Vis absorption spectra and electron spin resonance (ESR). The results showed that heteropolyanion, i.e. PW12O403- (PW12), maintained a Keggin structure in the film and there was a strong interaction between anion PW12 and cation R-NH3+; (R=link of hybrid composite). The photochromic properties of the composite film originated from the reversible charge transfer between the anions and cations. Under ultraviolet (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. Then the bleaching could occur when the film was in contact with ambient air or O-2 in the dark.