Based on aberration theory,the design idea of flat field unobstructed two-mirror system is presented.In this system,spherical aberration,coma,astigmatism and field curvature are free.Designing method and process is described and characteristic of system is summarized.Two systems are designed with the field of view of 6°×6° and 20°×1° and the image quality is in limit of diffraction.The configuration is simple and alignment is convenient.This system is very valuable for space sensing field.