The requirements on its load volume and weight are strict for unmanned airborne photoelectric pod. In order to meet the needs of 8-12.5 μm infrared detection, a lightweight and compact long-wave infrared optical system was designed and realized. The F number of the system was 2, the diameter was 150 mm, and the total field of view was 2.34 °. A twice focusing reflection-refraction structure was used as the optical system structure. It simplified the commonly used Cassegrain-type main system into a folded Newton-type main system, and simplified the spherical secondary into a plane mirror folding optical path. The off-axis aberrations were corrected by the aspherical correction lens group. An all-aluminum optical-mechanical structure was designed for the main system, combined with the optical-mechanical material matching of the rear optical path. In the temperature range of the working environment, the designed MTF of the system was more than 0.41@17 lp/mm, with 100% cold stop efficiency, and the volume was only Φ152 mm×125 mm. After final alignment, the MTF of the instrument was more than 0.24@17 lp/mm, and the image quality was clear and met expectations. The optical system was smartly designed, so the structure was light and compact, and the cost of manufacture and alignment was low. The design ideas and development methods can provide a reference for the optical systems of long-wave infrared instruments of unmanned airborne photoelectric pods for similar applications.