Abstract: The infinitesimal plutonium impurity was contained in the uranium product obtained by reprocessing. The precise measurement of the isotope ratio of plutonium was beneficial to the identification of the corresponding source of uranium product. Basing on the separation of trace plutonium from uranium matrix，the operating parameters of MC-ICP-MS were studied and optimized for determining the isotope ratio of trace plutonium, such as the adjustment of resolution, selection of mode of ion extraction, elimination of background and interference, etc. The correction method of the mass discrimination was explored for MC-ICP-MS measuring isotope ratio. The isotope ratio of ²³⁹Pu/²⁴⁰Pu in two selected uranium products obtained by reprocessing， which was measured and corrected by measuring plutonium reference material. The ratio of uranium and plutonium is more than 10¹⁰, and the sum of relative uncertainty for plutonium measurement is better than 5%. The measuring method of isotope ratiois establishes for ng•L^-1 level of plutonium.