The ultra large field of view infrared optical lens was mainly used to warn the incoming target militarily. Compared with the conventional infrared optical system, its design had many different characteristics. Combined with practical engineering applications, the characteristics of the design of the infrared field system of the large field of view were analyzed in terms of projection mode, optical configuration, image plane illumination, field of view, non-thermalization, evaluation mode and so on. A concrete design example was given. The detector adopted 1 024×1 024@ 15 μm cooled medium wave infrared detector. The operating band of the optical system was 3.7-4.95 μm, the focal length was 9.6 mm, the field of view was 116°, and only 4 lenses were used to realize optical non-thermalization without diffraction surfaces, the working temperature covered a range of -55-70 ℃, the lens structure was compact, and the total length was less than 70 mm. The results of image quality evaluation showed that the angular resolution uniformity of a single pixel in the full field of view was more than 95%, the energy concentration of a single pixel was more than 75%, and the illuminance of the optical system edge field of view was more than 90% of the central field of view.