The imaging principle of X-ray fluorescent tomography and its application in microanalysis field are introduced simply . Not only fluorescence but also various scattered light are produced while high energy X rays interact with substance. In order to eliminate the influence of these scattered light to imaging result, a method is proposed in which a doubly focusing crystal monochromator is placed normal to the accident X rays to separate X-ray fluorescence from the background and focuse it to a point on the detector surface. The method not only enhances the intensity of X-ray fluorescence but also permits the use of a smaller detector.