The dependence of the optical properties on the thickness of AgInSbTe phase change films is investigated. The reflectivity, absorptivity, thickness and optical constant of the films were measured. The change of the extinction coefficient and refractive index become significant when the film is very thin, and become larger at shorter wavelengths such as in blue and green region. These results are very useful in improving the accuracy of optical design and simulation of AgInSbTe phase change optical disks, as well as in the study of phase change optical disks at shorter wavelengths.