Abstract: Solubility of Pu(Ⅳ) was determined by oversaturation method under simulated disposal conditions. The underground anaerobic condition was simulated in glove-box. The solid-liquid separation was achieved by ultrafiltration. The plutonium radioactivity was measured by low background liquid scintillation spectrometer. The results show that the dominant oxidation state of plutonium either in deionized water or Beishan groundwater is tetravalent. The solubility of Pu(Ⅳ) in Beishan groundwater and deionized water is (2.8±0.9)×10^-8 mol/L and (1.6±0.8)×10^-9 mol/L. The specific ion interaction theory was used to calculate the thermodynamics constants under the experimental conditions. Either in deionized water or Beishan groundwater, the solubility-controlling solids phase is hydroxide plutonium (amorphous). In deionized water, the dominant species is hydroxide plutonium. In Beishan groundwater, the dominant species is hydroxide plutonium and Pu(OH)₂(CO₃)^2-₂.