A stereo projection optical system includes an image engine configured for providing light superimposed spatial information, a color selector positioned to receive a light output of the image engine, a transmission-type light modulator positioned to receive an emergent light of the color selector. The color selector is configured for selectively modifying the polarization of the light output according to the wavelength of the light output. The transmission-type light modulator alternates between a dark state and a bright state. From the foregoing, it will be apparent that the stereo projection optical system according to the present invention provides advantages in that its structure can be simplified with the reduction of its size by synthesizing lift and right image signals by displaying the stereoscopic image signal using a single projector.