Abstract: Hydrogeochemical processes in a saline water-freshwater transition zone was studied by mixing of saline water and freshwater in different proportions and simulation experiments of water-rock interaction. During the mixing of saline water and freshwater, mixing is the basis of the complex chemical processes based on mechanical mixing. The total alkalinity and pH values of the mixed fluids increase significantly, while the conductivity decreases significantly; the calcite and gypsum saturation indices increase significantly, while conductivity is reduced. In the course of the water-rock interaction, the pH values, total alkalinity and conductivities of the mixed fluids all increase significantly. The saturation indexes of gypsum increase from -1.06–-0.71 to -0.86–-0.50, indicating the dissolution of gypsum; the saturation indices of calcite are reduced from 0.69–1.07 to 0.00–0.10, indicating that calcite precipitates finally approache equilibrium.