Abstract: In nuclear industry, trace thorium in uranium product from mining and metallurgy is an important impurity. It is difficult to separate and analyze trace thorium in uranium product. It has important significance to improve of the quality of uranium product in our country that this problem is solved and applied to the production. A method was developed for determining the trace thorium in triuranium octoxide by ID-ICP-MS. The detection limit of this method is 0.003 3 μg/g（3s）, and s_r is less than 3%（n=3）. The results of the determination are in good agreement with the reference values in standard reference materials GBW04205, GBW04242 and GBW04243. These indicate that the method is a sensitive and accurate one and can meet the demand for trace thorium analysis in triuranium octoxide.