Abstract: The traditional unobscured three-mirror optical system is an intrinsically rotationally symmetric optical system with an offset aperture stop, a biased input field, or both of them, so off-axis sections of rotationally symmetric aspheric parent surface are ineluctable. Using the conclusion of vector wavefront aberration theory, a new unobscured three-mirror system by tilted the rotationally symmetric aspheric mirror was presented. The design reason and step of this system was analyzed, and then a system with effective focal length of 1 000 mm, field of view of 1020 and F-number 10 was designed. The volume of system(LengthWideHeight) less than 350 mm350 mm120 mm and image qualities of the example are near diffraction limit. Compared with other unobscured three-mirror system, the most prominent advantage of this system is that using tilted rotationally symmetric aspheric mirror to achieve unobscured style, thus reducing cost of the system.