Abstract: The speciation and solubility of americium in Beishan groundwater and Southwest China groundwater were calculated using a newly developed geochemical model CHEMSPEC by Peking University. Influences of pH, Eh, different ions and presence of Si on the speciation were investigated. The results indicate that americium exists as trivalent Am(Ⅲ) in the two different groundwaters and the species significantly changes with pH values. Under acidic conditions, americium mainly exists as AmSO⁺₄, Am(SO₄)^-₂ and Am³⁺. While in neutral or weak alkaline conditions, the main species is AmCO⁺₃. When the solution is in strong alkaline condition, most of americium exists as Am(OH)₃. When Si is present in the Beishan groundwater, the main species is AmSiO(OH)²⁺₃ in the pH range of 6.5-9.7. The concentration of different ions also have an influence on the speciation distribution of americium, and in the order of HCO^-₃>F^->SO^2-₄>Cl^-. The solubility of americium is greatly affected by pH and HCO^-₃ concentration. When pH value increases, the solubility of americium decreases. The solubility of americium in Beishan groundwater (pH=7.56, Eh=164 mV) and Southwest China groundwater (pH=7.50, Eh=0 mV) are 2.01×10^-7 mol/L and 1.60×10^-7 mol/L, respectively.