According to the requirements on high-resolution, wide-field-of-view, all-time reconnaissance and search for maritime targets, a coaxial catadioptric optical system with medium wave infrared (MWIR) is designed. It operates at the orbit altitude of 1200 km with the wavelength spanning from 3.7 μm to 4.8 μm, and it presents the ground resolution of 10 m at nadir. After detailed analysis and calculation, the focal length of this system is 3000 mm, and the relative aperture is 14. The angle of field reaches 14.203°, and the earth area coverage can reach 300 km with scanning former lens group. A focusing mechanism is developed to correct thermal aberrations of the system from 10 ℃ to 30 ℃. The modulation transfer function (MTF) corresponding to the whole field of view (FOV) is larger than 0.39, which can approach diffraction limit at Nyquist frequency of 21 lp/mm. In order to suppress stray radiations, the optical system is optimized to realize 100% cold-shield-match efficiency. The above results show that the properties and structure feasibility of this optical system closely meet the requirements.