Abstract: Based on the theory of coaxial three-mirror reflective optical system and by decentering the aperture stop and the field of view, a compact and multispectral off-axial three-mirror reflective optical system with 1 200 mm focal length was designed. Furthermore, the image quality was evaluated. The design results prove that the modulation transfer function(MTF) is above 0.5 at 80 lp/mm for 0.486 m ~0.656 m, above 0.35 at 17 lp/mm for 3 m ~5 m and the rootmean-square (RMS) radius is less than 2.096 m in full waveband. Moreover, three mirrors which use cone surfaces instead of high order aspherical surfaces are not decentered or tilted and only the aperture stop is decentered, so it is easy to fabricate and assemble.