Ocean Imager Optical system requires high spatial resolution in wide field of view.It's on the Geostationary Orbit,which is 35800km above the earth.Its spatial resolution needs to reach 250m in 2.46° field of view.The spectrum is 0.4~11.5μm,including 12 VNIR bands and 2 LWIR bands.In this paper,the design and result of an Off-axial Three Mirror Anastigmat optical system which met the requirements was described.The image quality reached the diffraction limit.