Thin a-Se films were prepared by thermal evaporation method. Optical transmission spectra and Raman scattering spectra were measured for stable a-Se film, unstable a-Se film and its initially spontaneous crystallization film. The transmission spectra were simulated and, refractive indexes varying with wavelength and thickness of films were calculated. The refractive index was increased during the spontaneous crystallization and, decreased with the increasing wavelength. The transmission intensity of the spontaneous crystallization of Se thin film was lower than that of the amorphous Se thin film and the absorption edge was shifted to longer wavelength. The distorted chain and incomplete Se8 ring structures were also enhanced during the spontaneous crystallization process measured by the Raman scattering measurement.