In order to meet the design requirements of multi-band, long focal length, large aperture and compact structure, the reflective optical system is studied. Based on the imaging theory of the coaxial three-mirror system, in order to avoid the existence of blocking in the optical system and improve the imaging quality, the design uses Zemax optical software to introduce a method to optimize off-axis three-reflection optical system. Compared with the control optimization based on ZPL language, the method is more convenient and intuitive. It does not need to use complex aspheric surface and only uses the conic coefficient of curved surface to meet the design requirements, which greatly reduces the cost and difficulty of processing and testing. Using the off-axis field-of-view, a visible and mid-wave infrared dual-band off-axis three-reflection optical system with a focal length of 1000 mm, the diameter of 6 in and a field-of-view of 2.2°×0.22° is designed. The simulation results show that MTF of the full field-of-view is greater than 0.4 at the spatial frequency of 17 lp/mm in the 3--5 m band, and MTF of the full field-of-view is greater than 0.7 at the spatial frequency of 50 lp/mm in the 0.4--0.7 μm band. The imaging quality of the system is good, close to the diffraction limit.