The invention discloses an infrared medium-long wave double wave band imaging optical system which is mainly used for an infrared focal plane imaging system. The invention puts forward a tetra-sheet optical system. A light beam coming from an infinite distance object space in order passes through a first lens, a second lens, a third lens, a fourth lens, a Dewar window and a aperture diaphragm to form an imagining on an image plane. The aperture diaphragm of the system superposes the Dewar cold diaphragm of a refrigeration photoconductor. According to the system, medium-long wave infrared (MWIR, 3 to 5 [mu] m) and long wave infrared (LWIR, 8 to 12 [mu] m) wave bands form imaginings on a same focal plane. In the system, the relative aperture is 1 to 3, the focal length is 300 millimeters and the field of view is 8 degree. The structure of the invention is mainly characterized by double wave band work, common infrared materials used in the system, 100 percent diaphragm efficiency, simple structure and easy assembling.