Abstract：The knowledge of speciation is the key contents of performing safety assessment studies for high-level radioactive waste repositories. As the main elements of high-level radioactive waste，three radionuclides，Np(IV)，Pu(IV)，Am(III)，are calculated by using EQ3NR of geochemical model EQ3/6. The main speciation of the three radionuclides，Np(IV)，Pu(IV)，Am(III)，are NpO2＋，Pu(OH)5－，AmCO3＋，Am3＋，Am(OH)2＋in the underground water. This paper also studies the speciation of the three nuclide elements，i.e. Np(IV)，Pu(IV)，Am(III)，under different conditions such as various pH values，the concentrations of Cl－，F－，SO42－and HCO3－. The study illustrates that increase or decrease of pH values will affect the variety of the speciations of Np(IV)，Pu(IV)，Am(III)，and the concentrations of Cl－，F－，SO42－and HCO3－have the less impact on the speciations of Np(IV)，Pu(IV)，Am(III).
