A stereo projection optical system includes a first polarizing beam splitter, a transmission-type light modulator positioned to receive the first polarized light component from the first polarizing light splitter and an image assimilator positioned to receive an emergent light of the transmission-type light modulator. The image assimilator includes a second polarized light splitter and first, second reflective spatial light modulators. The stereo projection optical systems provide viewers three-dimensional images formed by two alternative polarization light beams whose polarizations are perpendicular to each other utilizing the transmission-type light modulators to form “3-D” images.