Abstract: A low F number long-wavelength infrared optical system with wide field of view is designed. It consists of three aspherical germanium lenses with a reversed telephoto telecentric structure. The effective focal length is 6mm, the relative aperture is 1:0.8, the full field of view is 160, and the total optical length is 86.13mm. The result shows that the modulation transformation function is greater than 0.7 at the spatial frequency of 20lp/mm. The system inherits the advantages of the reversed telephoto telecentric structure, it is simple in structure and small in volume, light in weight, uniform in illumination, and also has large field of view.