A cooled long-wavelength infrared imaging optical system is proposed and designed for a 320×256 long-wavelength refrigerated area array detector. The optical system is composed of five lenses, and the system is designed with different material combinations and a back-focus adjustment mechanism to achieve clear imaging in the operating temperature of －40℃ to 70℃. The working spectrum of the optical system is 7.5-9.5 μm. The focal length is 50 mm; the relative aperture is 1/2; and the full field of view is 11°×8.8°. This system has the advantages of a simple structure, large relative aperture, and high transmittance. Design results show that the modulation transfer function (MTF) of the optical system is better than 0.594 at a Nyquist frequency of 16.7 lp/mm, and the root mean square size is smaller than the single pixel size. Energy concentration is better than 88.5% within the pixel size, and distortion is less than 0.23%. After setting a tolerance, the MTF of the system was better than 0.504, indicating that the system is easy to process, has high realizability, and exhibits good imaging performance after assembly.