The way to expand the field of view (FOV) of an optical system by using the principle of image rotation scanning is analyzed. An optical design model which uses a secondary imaging structure is established. By using a Pechan prism as a one-dimensional scanning device and tilting a cooled mid-wavelength infrared detector, the exit pupil of the optical system and the cold stop of the detector are fully matched. In addition, to ensure the imaging quality at different temperatures, a passive optical athermal technology is used. The model has a relative aperture of 1∶3, a wavelength of 3.7 μm to 4.8 μm, a focal length of 34 mm and a FOV of ? 20?. By using eight lenses among which two are aspheric, the imaging quality close to the diffraction limit is obtained in the whole FOV.