The coaxial three-mirror optical system has the advantage of small volume, easy to assembly and high quality image, which can be widely used in the field of aerospace remote sensing. The initial configuration parameters of the system are resolved by using the primary aberration theory, a coaxial field-bias three-mirror optical system with a focal length of 25 m and F number of 12.5 is designed. The design results show that the field of view of the system can attain to 0.6°×0.3° by offsetting the rectangle field with low parasitic light and compact structure, by using the folded mirror, the total length is about f′/6.0~f′/6.6, which is suitable for linear array time-debyed-integration charge coupled device (TDI-CCD) sensor to push-scanning image. The modulation transfer function (MTF) is higher than 0.47 at 50 lp/mm and the image quality of the optical system approaches the diffraction limit. The optical system provides a good option for the field of high-resolution space-to-earth fine observation.