To improve the calibration accuracy of X-ray detectors, this paper presents a method of placing filters in fluorescent X-ray emission channels to improve the purity of X-rays. Monte Carlo simulation model was established to analyze the relationship between the probability of photoelectric effect in K layer and the atomic number, and the curve of fluorescence intensity and purity with filter thickness was obtained. In atmospheric environment, the energy spectrum distribution and photon flux of fluorescent X-ray source were measured by silicon drift semiconductor detector, and the effect of X-ray tube voltage on photon flux and fluorescence purity was analyzed. When the radiator material is copper and the thickness of the filter (nickel) is 0 μm, 10 μm and 30 μm, the purity of fluorescence X-ray measured is 75.61%, 85.38% and 84.25%, and the photon flux is 3425 phs/s, 2023 phs/s and 1192 phs/s, respectively. The influence of filter thickness on the purity and intensity of fluorescent X-ray is confirmed, which provides a direction for solving the problem that it is difficult to calibrate X-ray detectors with high accuracy due to the lack of monochromatism of fluorescent X-ray light source.