Abstract: ²³⁷Np is one of the key radionuclides for the safety assessment of high-level radioactive waste repository. The environmental chemistry of neptunium is concerned. In this work, the solubility of neptunium in Beishan groundwater was analyzed by CHEMSPEC. The influences of redox potential Eh, pH, and the total carbonate concentration on solubility were investigated. The results indicate that the solubility range of neptunium in Beishan groundwater is 8.19×10^-8-3.29×10^-3 mol/L. Neptunium would precipitate in the form of NpO₂ and the solubility would be lower in reducing environment. The precipitation would change to be NaNpO₂CO₃ or NpO₂OH in oxidation environment and the solubility would be higher. The influence of pH on solubility depends on the speciation of precipitation. For NpO₂ and NpO₂OH, the solubility would increase with the total carbonate concentration at pH=7.25. For NaNpO₂CO₃, the solubility of neptunium would decrease with the increase of the total carbonate concentration at pH=7.25.