A novel photochromic hybrid film containing 12-tungstophosphoric acid (PW12) and dextran has been successfully prepared by solution casting method. The Fourier transform infrared (FTIR) and ultraviolet-visible (UV-Vis) spectra revealed that the Keggin geometry of the PW12 was still preserved in the hybrid film and a charge-transfer bridge was built between PW12 and dextran through hydrogen bonds. Wide-angle X-ray diffraction (WAXD) was used to investigate the morphology of the hybrid film. Under UV irradiation, the hybrid film was reduced photochemically to yield a blue species. The reversibility of the photochemical reaction of the hybrid film was observed. The characteristic signal of W(V) in the electron spin resonance (ESR) spectrum and the changes in the UV-Vis and FTIR spectra after irradiation was identified, indicating that PW12 was reduced to one-electron heteropoly blue and partial hydroxyl groups of dextran were oxidized to the carbonyl groups. A conceivable mechanism for the photochromic reaction was also proposed.