This paper describes the results of the design effort for a new kind of three-mirror anastigmatic system (TMAS) and its tolerance exercise. The system’s focal length is 100mm and the relative aperture is f/10, its total length is nearly f’/3~f’/3.5 and field of view (FOV) is 10° x 1°. The final optical system is a three-mirror unobscured telescope. It achieves diffraction-limited imagery at visible wavelengths. Finally, we can draw two conclusions: 1) design of unobscured optical system is still best approached using fundamental optical design principle; 2) time spent in careful modeling of fabrication, testing will result in more relaxed tolerances and a higher probability of success for the assembled system.