Based on the principle of optical offsetting image rotator, image rotator scan is studied to expand the optical system Field of View (FOV). An optical design model is introduced, which is a triple imaging zoom structure, and Pechan prism is used as a one-dimension scan component. Moreover, the active athermalization technology is used to satisfy the imaging quality in different temperature. The optical design result is given. The system uses a 480ˉ6 linear array detector of long wavelength infrared (7.7 ~10.3 μm), the detector length is 12.29 mm, and the F# is 2.6. The detector is tilted and decentered to match the cold stop, and the image rotator scan makes the model have a circular FOV whose radius is the detector length and the system FOV is 4.4o/17.6o with 320/80 mm focal length. The system has 9 lenses with 3 aspheric surfaces, and has near diffraction limited imaging quality in whole FOV.