In order to realize the round-the-clock space-to-earth observation with high resolution, a visible and infrared bands common aperture optical system is designed, taking full advantage of the high resolution of the space optical system with long focal length and large aperture. The system consists of a visible spectrum optical system and an infrared (IR) subsystem. The fields of view for the two spectral ranges are both 1.3°×0.1° which is convenient for push-broom imaging simultaneously. A coaxial three-mirror structure with a middle image plane and a real exit pupil to facilitate suppressing the stray light is utilized by the visible spectrum optical system whose working wavelength band is 0.38~0.78 μm, focal length is 25 m, and F-number is 10. The beam is split near the exit pupil of the visible spectrum optical system, to which an IR subsystem is designed and connected. Then the coaxial threemirror optical system and the IR subsystem are integrated into a common aperture IR optical system, whose working wavelength band is 8~10.5 μm, focal length is 4 m and F-number is 1.6. After optimization, the structure of the whole system is compact, and the image quality in the two bands approaches the diffraction limit satisfying the design requirement. The tolerance analysis of the two systems is accomplished, and the result of Monte-Carlo analysis indicates that a static modulation transfer function(MTF) better than 0.2 is achievable.