Based on the third order aberration theory, a merit function is established, genetic algorithm is employed to find the initial structure parameters with minimum merit function then the initial structure is loaded to ZEMAX for further optimization. A telecentric three- mirror reflective system with effective focal length of 1700 mm, entrance pupil diameter of 200 mm and field of view of 18°×0.6° is designed by making the yfield off axis. The performance of the system is close to the diffraction limit and it is unobstructed and suit for the field of space optical remote.