Abstract: For the disadvantage of conventional three mirrors system to achieve wide field of view, a new-style unobscured three-mirror optical system with wide field of view was designed based on the retrofocus-type objective configuration. One powerful retrofocus design approach was the combination with a reversed Galilean Telescope, the expression of initial configuration parameters was obtained and the unobscured style was carried out by decenter the field. A system with effective focal length of 100 mm, field of view of 2020 and F-number of 5 was designed, the modulation transfer function value at spatial frequency of 50 lp/mm was over 0.6 and image qualities were excellent. Compared with traditional common three-mirror optical systems, the most prominent advantage of this new three-mirror system are that it can provide wide field of view and just use two aspheric mirrors and one spherical, so that the cost of this system will be reduced.