An azo dye-doped polymer thin film was made by a spin-coating method on single-crystal silicon substrates. The complex dielectric function and optical parameters of the film have been measured with a rotating-analysis spectroellipsometer and an absorption spectrometer. The results show that the film has a strong and broad absorption in the wavelength region of 400～600 nm, and the aggregation state of the dye molecules has a greater influence on the absorption spectrum.