In order to meet the requirements of miniaturization of thermal imaging system and different environmental conditions in the electrooptic POD of micro-mini UAV, an uncooled infrared imaging optical system with field of view of 24.5°×19.7° was designed by means of optical passive athermalization, reasonable distribution of focal degrees of lenses with different materials and introduction of a binary optical element. Its operating wavelength range is from 8 m to 12 m， F number is 1.0. The system consists of four lenses with a total weight of only 35 g， and the total length of the optical system is 38 mm. The result shows that the system structure is simple and compact, the back working distance is large, and the imaging quality is high. The modulated transfer function (MTF) value at the cutoff frequency of 30 lp/mm is greater than 0.3, which satisfies the application requirements.