A hemisphere long wavelength infrared (LWIR) imaging optical system design is introduced. The secondary imaging structure is adopted in the system to compress the radial size of the system effectively. For wider field of view of the system, F-θ dissimilarity imaging mode is adopted. Optimization design is carried by using ZEMAX software, so the problems such as image illumination uniformity and wide field of view off-axis aberration are solved. At the same time the system is analyzed and evaluated and design results such as transfer function curves and spot diagram are presented. Only 6 pieces of optical components and 2 pieces of aspheric surface are used in the system. The full field of view and relative aperture is 184° and 1/1.93 respectively, the transfer function value in full field of view is more than 0.45 at 20 lp/mm, and good image quality is got. The total length and the longest diameter of the system are 71 mm and 30 mm respectively, the weight of the optical components is only 45 g. The optical system has small volume and light weight, which is suitable for a variety of platforms.