Based on the third order aberration expression, basic design process of four-mirror optical system with aperture stop on secondary mirror are given through aberration analysis. Then a system with effective focal length of 1343 mm and field of view of 20°×0.6° is designed by making the field of y-direction off axis. The structure of this system is unobstructed and tight, with imaging qualities approaching diffraction limit. The designed system is suit for the field of space optical remote sensing.