Abstract: An Environmental Radiochemical Speciation Analysis Code (ERSAC) using the thermodynamic data of elements in water solution, based on the concentration of a radionuclide in a waster system, seeking the possible species of the radionuclide produced by various chemical reactions in the water system and the related equilibrium coefficients of different species, calculates the possible species of the radionuclide and their equilibrium concentrations and provides a speciation profile of the radionuclide in the water system. This paper presents the calculation result of the solubility of neptunium(Ⅵ) in Beishan well 51^# groundwater and the influences of pH and groundwater compositions on the solubility using an environmental radiochemical speciation analysis code CHEMSPEC which is developed by the Nuclear Environmental Chemistry Group of Peking University. Our calculation indicates that pH and total carbonate concentration do affect the solubility of neptunium while the sulfate concentration and chloride concentration don’t. The solubility range of neptunium(Ⅵ) in Beishan well 51^# groundwater is from 1.1×10^-5 to 4.4×10^-3 mol/L.