Abstract: The development of aerospace science paves the way for improving earth observation and the optical imaging remote sensing technology is developing rapidly. The imaging quality can be improved by improving remote sensor quality, enlarging focal length, increasing record medium resolution, and improving optical system transfer function. A threemirror anastigmatic optical system, whose focal length is 36m and field angle is 0.75°, is introduced. The system can realize instantaneous space-to-ground scanning for a range of 90km×90km, and staring imaging over an area of 900km×450km when the system is swinging. The result of the system design is given.