One new-style unobscured three-mirror optical system was designed,which was based on the Gregorian system. The design step of this special three-mirror optical system was analyzed,and then a system with effective focal length of 900 mm,field of view of 0.8°×0.8° and F-number of 9 was designed. Its total length is 330 mm. The image qualities of the example are near diffraction limit. The system has advantages of shorter physical size and better ability in restraining stray lights. Compare with other three-mirror optical systems,the most prominent advantage of this special three-mirror optical system is that it just uses two aspheric mirrors and one spherical,thus reducing the cost and the manufacturing difficulties a lot.