In order to track and capture target at the same time with a set of equipment, a middle infrared spectrum variational field of view(fov) detection system with a large focal plane array was designed for a 640×512 novel large focal plane array infrared detector with the picture element size of 15 m×15 m. The spectrum range was 3.7～4.8 m, F number was 4.0, narrow FOV and wide FOV was 0.45 and 0.90 respectively. The manner of variational FOV was accomplished by pitching two lenses into the narrow FOV system layout with mechanism framework. Reimaging technology not only minished the diameter of front fixed group, but also met 100% cold shield efficiency to minish stray light into the infrared detector. Two common infrared materials Ge and Si were used in the variational FOV detection system. The aspheric technology was used in order to correct the off axis aberration and higher order aberration and assure the fixedness of image plane. At the spatial frequency 33 lp·mm-1, the modulation transfer function(MTF) was above 0.2 for both the narrow FOV and wide FOV. Moreover, the distortion is below 0.5%. The middle infrared spectrum variational FOV detection system has excellent image. The image quality of the middle infrared spectrum variational field of view detection system changed little in the working temperature range -35～55 ℃.