The thickness, refractive index and transmission loss of films are important parameters in determining electro-optical coefficients and designing optical waveguide devices. The guest-host polymer disperse red 13 (DR13)/poly-methylmethacrylate (PMMA) thin films with three different concentrations were prepared by the spin-coating method, whose absorption spectra were obtained using a spectrophotometer. Their refractive index and thickness were measured with a prism coupler and the dispersion curve was plotted by fitting their refractive indices at different wavelengths. The experimental apparatus for an imaging technique were set up, by which the films′ transmission losses were measured with the help of image processing program written by ourselves. From the absorption spectra, there are two large absorption peaks at about 300 nm and 500 nm and no absorption peak in the telecommunication wave band, so this material can potentially be used in telecommunication devices. Thicknesses of the films are about 1～2 μm. It can be seen that at the same input wavelength, the refractive index becomes greater as the mass ratio increases, and when the wavelength of the input laser increases, the refractive index of the same mass ratio reduces. The error values of thickness and refractive index of the films are about 3.2×10-1 μm and 1.5×10-3. The transmission losses of three different mass ratio (10%, 15%, 20%) are 1.5269 dB/cm, 2.7601 dB/cm and 3.6291 dB/cm, respectively. It is shown that the transmission loss increases as the mass fraction increases, that is, DR13 is more important for the transmission loss than PMMA.