Separating a single rare earth element (REE) from the rare-earth concentrate is a continuous process, in which the artificial sampling - ICP laboratory analysis is usually used for process monitoring and control. The results of the test lag behind the actual production and may lead to problems such as the instability of the product quality and so on. Based on the energy dispersive X-ray fluorescence spectrometry, a method to determine the REE distribution in the process of rare earth separation was established. The X-ray fluorescence spectral signal of single rare earth element was obtained by multiple stepwise regression via analysis of typical north rare earth elements (lanthanum, cerium, praseodymium, neodymium). The experimental conditions, such as the filter, tube and tube flow, were optimized according to the relative theoretical deviation, which laid the foundation for online analysis of REE distribution. In this study, the XOR-50 analytical equipment and on-line detection method of rare earth distribution were developed, which could fast reflect the technical condition of rare earth extraction and separation, providing real-time online extraction data and offering accurate and reliable data for process adjustment. The results showed that using 0.2 mm of Al filter, 25 kV of lightpipes excitation voltage, 1 100 μA light tube current test conditions, the relative standard deviation of the REE distribution in the same sample for 11 consecutive times was less than 1%. The results of field analysis were consistent with ICP-AES test results. The detection limit of rare earth elements, such as lanthanum, cerium, praseodymium, neodymium and other light rare earth elements, was detected to be less than 5 μg·mL-1, which completely met the accuracy and reliability requirements of rare earth distribution online monitoring.