An optical device includes a waveguide device, a reflective-type light valve and a projection lens. The waveguide device receives a first polarized beam and includes a first surface, a second surface and the first grating. The first grating is disposed in a path of the first polarized beam to change a propagation direction of the first polarized beam, and the first polarized beam passes through the first surface, the first grating and the second surface in succession. The reflective-type light valve is disposed downstream from the second surface of the waveguide device to convert the first polarized beam into an image beam. The projection lens is disposed downstream from the reflective-type light valve, and the image beam passing through the second surface of the waveguide device, the first grating and the projection lens in succession.