According to the requirements of high imaging quality, fast F-number, miniaturized dimension and wide environmental adaptability in the low cost, small size and long focal length uncooled thermal imaging systems presently, several compact optical configurations with fast F-number and small obscuration are given. Based on catadioptric structure, a fast F-number compact long infrared optical system with relative aperture of 1/0.89 and telephoto ratio of 0.67 is designed. This optical system can be used for the low-cost uncooled compact infrared imaging system. Optical passive athermalization at -40 ℃~60 ℃ temperature range is realized by using athermalization design method. Stray light influence of the optical system is also analyzed. The result shows that the optical system works well through entire operating temperature range, and obtains the advantages of compact structure, small volume and so on. It can meet the requirements of the small size long focal length uncooled thermal imaging systems.