Abstract: Since the refractive index of media in the X-ray region is lower than 1 and Xray is strongly absorbed by media, it is very difficult to image objects in the Xray region. Conventional imaging methods are not suitable to X-ray wave band. Generally, the methods of the grazing-incidence reflective imaging and coding aperture imaging are often used. Imaging quality of a single mirror and two mirrors in the case of grazing incidence is discussed. The focal length, field obliquity and diaphragm position of KBA microscope are analyzed. The results show that the field obliquity is the minimum when the grazing-incident angle is half of the angle between the two mirrors, and the imaging quality is better when the diaphragm is placed on the second mirror instead of the first one. The reasonable structure arrangement of KBA microscope is analyzed. The theoretical basis is provided for the design and manufacture of KAB microscope.