Aiming at 640×512 long-wavelength infrared cooled detectors, a cooled long-wavelength infrared optical system was designed to track and detect an infrared target. The optical system adopts the secondary imaging structure to ensure 100% cold-shielding efficiency, and adopts a combination of optical material Ge and ZnS to achieve aberration correction and achromatic design. By introducing the high-order aspheric surface, the high aberration of the system is well-corrected, thus the system structure is simplified. The optical system is composed of 6 lenses. The focal length is 400 mm, the working bands are 7.7~9.3 μm, the field of view is 1.37°×1.10°, and the F-number is 2. The design results show that at a spatial frequency of 33 lp/mm, the MTF of off-axis field of view is more than 0.24, which approaches the diffraction limit and has high imaging quality. In the operating temperature range of -35~+55 ℃, the focusing lens is used to ensure the imaging quality under high and low temperature environments, which can be used for infrared tracking detection over a wide range of temperatures.