As per the requirements of high imaging quality, large relative aperture, lightweight, and wide temperature adaptability of the non-cooling thermal imaging system, based on the structural analysis of the lightweight optical system, the initial power distribution of the system is solved using the principle of optical-passive compensation. Moreover, a lightweight long-wave infrared athermalization optical system is designed using the characteristics of binary diffraction surface different from infrared materials. The focal length of the designed system is 50 mm, and the F-number is 1. It is suitable for a long-wave (8‒12 μm) infrared uncooled detector with a resolution of 640×512 and pixel size of 12 µm. The optical system comprises three lenses having a total mass of 67 g. In the temperature range of -40 °C‒60 °C, the modulation transfer function of each field of view is close to the diffraction limit. The experimental results of room temperature demonstrate that the designed system has a good athermalization effect.