Abstract: The Eh for reduction of U(Ⅵ) to UO_2 in groundwater of a radioactive waste disposal site in the Southwest of China was calculated with Nernst equation,and the(various) factors influencing the Eh were discussed.The results show that the Eh for reduction of U(Ⅵ) to UO_2 in the groundwater is(-0.177±0.011) V,and the values are obviously affected by pH,concentrations of U(Ⅵ) and CO~(2-)_3.The thermodynamic(model) of groundwater could be judged by calculating Eh values of some Ox-Red.