Abstract: The design of optical systems with wide ground coverage are restricted by the size of infrared detector. This problem can be solved by choosing the appropriate imaging mode. A kind of image space scanning method was given in the paper. It could satisfy the optical systems with general area infrared detectors. The picture was built by image mosaics technology. Because the image space scanning method needs to be done in the parallel light path, the design method of three-mirror afocal system was studied based on two-mirror afocal system and the formulas to compute the initial structure was given. The optical system consists of afocal system, scanning mirror and imaging part. The scanning mirror was placed at the exit pupil of the afoacl system. The MWIR and LWIR were separated by the field-bias method and imaged respectively. The simulation analysis shows that the Narcissus is under control and the MTF of the optical system is very close to the diffraction limit.