Abstract: Clastic quartz is the main mineral component of the eolian loess. The Crystallinity Index(CI) of clastic quartz is related to the type of mother rock, and the quartz content(QC) can be used to estimate the dust flux change so as to restore the desertification history of the source area. The paper discussed in details the testing conditions and data processing methods, and the errors they may cause to the estimation of QC and CI.It is concluded that (1) The scanning step size and speed should be different when CI and QC are measured separately; (2) Before the data processing, the background should be deducted manually. CI was estimated by the diffraction peak value at 67.74° and the diffraction valley value at 67.85°, and the estimation of QC mainly depends on the diffraction quartz peak value at 20.9° and diffraction silicon peak value at 28.4°; (3) CI should be the average of repeated measurement by pressed methods.