Abstract: In order to make the measurement results more scientific, the evaluation of uncertainty is more and more important．During the process of precious metal content tested by using the X-ray fluorescence spectrometry, the measurement system is based on the standard curve and the results are given directly by the equipment, which could be equivalent to the direct measurement．Taking the sample Au750 for example, the concrete evaluation process is discussed．The results show that the main sources of uncertainty is related to the measurement system, including the given value and error of the standard sample, the resolution, accuracy and calibration uncertainty of the equipment, which ignore environment factor and improper operation of human factors．The evaluation process is simplified and the method is rational．