Optical films are prepared by using ion beam sputtering deposition. Basing on the spectroscopic ellipsometry measurement technique, the relationship between refractive index, film layer thickness, surface layer thickness and spot size is studied. The study results show that, with the increase of tested spot size on the sample surface, the refractive index of thin films decreases, while the film layer thickness and surface layer thickness increase. The spot effects of optical constants are verified by using the reflectance spectrophotometry and the profilometer, respectively. The study results show that the refractive index and the film layer thickness have weak transverse inhomogeneity, which can be weakened by the usage of large-size tested spots.