A polarization modulator for stereoscopic projection comprises a polarization beam splitting prism assembly for splitting an incident beam into a transmitted beam, an upper half of reflected beam, and a lower half of reflected beam, a polarization plane rotating component for rotating the polarization plane of the transmitted beam or of the upper half of reflected beam and the lower half of reflected beam by 90 degrees, a reflective mirror for adjusting a propagation direction of the upper half of reflected beam and the lower half of reflected beam, a lens group for adjusting the range of size of the transmitted beam, a linear polarizer for filtering the beam, a polarization modulator for modulating the transmitted beam, the upper half of reflected beam and the lower half of reflected beam into counter-clockwise circularly polarized light and clockwise circularly polarized light in the order of frames, and a driving circuit.