This paper investigates that polarizations of a laser vary in the process of reflections after a linearly or circularly polarized beam is incident on a paraboloid of revolution-shaped cavity. This analysis is based on the viewpoint of geometry optics because the opening radius is about 100μm, which is ten times the size of the wavelength 10.6μm of an incident CO2 laser beam. Considering the profile of laser beam to be Gaussian distribution, the variation of polarization in the process of reflections is mathematically modeled. The result reveals that the state of polarization for an incoming ray with circular polarization is almost totally transformed into linear polarization perpendicular to the plane of incidence after the ray intervenes multiple reflections. The effects of cavity depth and absorptive index of material on the polarization variation are also discussed. The circularly polarized ray entering the deeper cavity is finally changed into the higher degree of linear polarization. The increase in absorptive index reduces the speed approaching the linear polarization.