Optical thin films exhibit inevitable polarization effects at oblique incidence. When illuminating light with a large numerical aperture incident on dichroic thin films at a non-normal angle in Philips prism system, linear polarized light is turned into elliptical polarized light because of the difference of the phase shift between s- and p-polarization component. Polarization aberration in projection display system is analyzed based on the theory of Jones matrix polarization ray tracing. The dark-state output and extinction ratio of system are also calculated. Furthermore, the character of polarized light on the exit pupil is given out, and the variation of polarization aberration at different F-number illumination light is analyzed. At last, the influence of monitoring errors of thin film on the polarization aberration is analyzed. The monitoring errors have great impact on the polarization aberration and contrast of the system. The method can be applied to other large aperture optical systems.