In reflective liquid crystal light valve (LCLV) or LCOS light valve large screen projection display system, Philips color separation and color recombination prism system is widely used to make the projection system compact. There are two factors which influence the contrast of projection optical systems. One is the extinction ratio of Polarizing beam splitter (PBS) and the other is the dark-state output of Philips prism system because of geometrical polarization rotations. Optical thin films exhibit inevitable polarization effects at oblique incidence. Here, Geometrical polarization rotation is analyzed in detail. The factors affecting the contrast performance of Philips prism system are quantitatively analyzed. Polarization aberration in this display system is analyzed based on the theory of Jones polarization ray tracing. It can be conclude that the contrast of projection display system is intimately correlated with phase shift difference of the two polarization component. It is concluded that optical design should combine lens design and coating design to improve the contrast and image quality of the projection display system.