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A new method for the determination of light element phosphorus in feed by energy dispersive X-ray fluorescence (EDXRF) analysis was proposed. An indirect measurement was applied, and the target elemental phosphorus was converted into quinoline phosphomolybdate by a series of chemical reactions. After washing, filtration and drying, the content of molybdenum element in the sample was measured by EDXRF. And according to the relationship between the phosphorus element and the molybdenum element in the chemical products, the content of phosphorus can be calculated. The optimum determining conditions of molybdenum element were also investigated. By calculating the peak-to-back ratio of character line of molybdenum element, the relation curve was drawn. The optimal tube voltage of molybdenum element was 27kV, the optimal tube current was 9μA, and the best measuring time was 120s. In this experiment, the content of phosphorus in phosphoruscontaining feed was 22.36%, the relative standard deviation was 0.46%, and the relative range was 0.24%. The consequence was in accord with the result of the melting method (X-ray fluorescence analysis). The analytical method of combination of physics and chemistry was put forward, some problems such as low detection efficiency and poor measurement accuracy were conquered, and the results showed that the experiment had good accuracy, stability and repeatability. The method provided a new way for measurement of phosphorus in feed, and showed good reference and directive significance for the determination of other light element content by EDXRF as well.