The invention discloses an 
isotope dilution mass spectrometry method for determining the content of 
uranium in a 
uranium niobium alloy. A formula for calculating the content of 
uranium in the uranium 
niobium alloy is derived according to an 
isotope dilution mass spectrometry principle. A 
decomposition process of the uranium 
niobium alloy is researched, the sampling quantity and the amount of a 
diluent are optimized, and the influences of 
mass spectrum line interference and 
alloy element interference on a measurement result are discussed. The method comprises the following steps: adding 
nitric acid and 
hydrofluoric acid to quantitatively dissolve the uranium 
niobium alloy, adding a quantitative amount of a uranium 
isotope diluent to directly prepare a mixed sample solution, determining the 
mixed solution and the uranium isotope proportion in the uranium 
niobium alloy sample through 
mass spectrometry, and calculating the content of uranium in the uranium 
niobium alloy. Quantitative separation of uranium is not needed by the determined method. An XRF technique, an ICP-AES technique and an 
element analysis technique are used to measure the content of niobium and the total content of 
impurity elements in the uranium niobium alloy, and 
back stepping is carried out to obtain the corresponding uranium content order in order to verify the accuracy of the analytical result, and the obtained result is consistent with a result obtained through the experiment method. When the method disclosed in the invention is used to analyze the uranium niobium alloy sample, the relative 
standard uncertainty of determination results is 0.2% (6 determinations), and the expanded uncertainty is 0.5% (95% confidence level).