Abstract: By comparing the advantages and disadvantages of two-mirror and three-mirror reflective systems,a novel unobscured two-mirror three reflective optical system was presented. In contrast with other off-axe three- reflective optical systems, the greatest advantage of this system was that it only utilized two aspherical reflectors,which saved the cost,reduced the difficulty of process. The design steps of this system were analyzed in detail. And a optical system was designed with its focus length, field of view, relative aperture, and system length are 500 mm, 10.1, F/5,125 mm respectively. Pixel size of the system was 10. Its MTF curve and spot diagram show the imaging quality approximates its diffraction limit. This approach can provide reference for the optical system design of present aerospace cameras.