Abstract: The rare earth element was separated from U_3O_8 by the cationic exchange column after U_3O_8 sample was resolved in the nitric acid. The recovery rates of the rare earth elements extracted from U_3O_8 were 98%~101%. The detect limit of the rare earth elements was o.1 μg/g(3σ, n =6). The optimization of the separating factor and instrumental parameter, and the effect of the residual uranium amount on measuring rare earth element was discussed in great detail.