Small satellites had the advantages of low-cost and being suitable for flexible tactics application. But it was a challenge to realize optical remote sensing in meter or sub meter level based on satellite platform which weight dozens of kilograms. A new two-mirror catadioptric system which consisted of two aspheric mirrors and a negative focal power group with three lens was proposed. This system was coaxial as traditional two-mirror system. Then the structure was compact and installation would be easy. At the same time, the system had flat filed and was anastigmatic as three-mirror system. At first, basic idea and structure of the new system were introduced, its primary aberration formula was derived, the means for correcting its aberrations including chromatic aberration was analyzed and presented, and thereby its initial structures were solved. Then, An example which had compact structure and was suitable for optical system on low-orbit high-resolution small satellite was designed and optimized. Its work band, full field of view, effective focal length and F-number were from 450 nm to 800 nm, 1.2毅, 3000 mm, F/10. Length of the optical system and the stop were 595 mm and 300 mm. Negative focal power lens group corrected chromatic aberration by itself, and three lens used same common glass. Imaging quality of the system was diffraction-limited.