Abstract: Based on the principle of photomicrography, image space scanning is studied to be as the method to expand the optical system-s field of view (FOV). An optical design model is introduced, which has secondary imaging structure, including a large FOV fixed front objective lens set and a spherical motion relay lens set. The field curvature of the first image plane of the front lens set is optimized, and the motion trochoid of the relay lens set is according to the curve radii. The relay lens set image the different fields of the first imaging plane and get the image space scanning. Optical passive athermaliztion technology is used to ensure the system imaging quality at different temperatures. The model relative aperture is 1∶3, the wavelength is 3.7m~4.8m ,the focal length is 90mm, the instantaneous staring FOV is 4, and the scan FOV is 24. The whole optical system has 7 lenses with 3 asphere surfaces, which is approximate to diffraction limit over the full FOV.