The nanocrystalline TiO_2 thin films plays an important role in the photoelectrochromic devices. The microstructure of nanocrystalline TiO_2 has effects on the properties of absorption of dyes, diffraction of light and transportation of charges. Nanocrystalline TiO_2 thin films were prepared by the electron beam evaporation. The surface morphology, crystal state and composition were analyzed with atomic force microscope(AFM), X-ray diffraction(XRD) and Auger electronic spectroscopy(AES). The photoelectric performances affected by particle size were discussed theoretically. And the blue-shift phenomenon of the absorption peak value was explained by quantum confinement effect.