In this paper, a method for quantitative determination of spectral purity of X-ray tubes for diffraction analysis is presented. The instrument used for this purpose was a home-made diffractormeter with a monochrometer of quartz crystal plate which is used as the analyzer. The experimental values of the X-ray intensities of various wavelengths must be expressed in terms of intensities just emitted from the X-ray tube window. Theoretical analysis of the various factors affecting intensity was made in detail, and the conversion factors for the reduction of intensities corresponding to different target elements and impurities was summarized in a table. The intensity of major characteristic spectrum line of the target element was attenuated by Cu or Al absorption foils, so as to avoide the errors caused by counting loss. Using this method, the spectral purity of a number of X-ray tubes was determined.