Abstract: On the basis of hydrogeologic investigation, groundwater,sea water and the medium are collect-ed, whose composition and nature are determined, and then the regularity of permeability(penetration coefficient, porosity and dispersion coefficient) and the main ion(Na⁺, K⁺, Ca²⁺, Mg²⁺) are determined in the course of salt water-freshwater displacement with transfusion installment. Finally, hydrogeochemistry is studied for the displacement with the numerical simulation. The result indicated that ion exchanges take place in the process of salt water-freshwater displacement, the density of Ca²⁺ and Mg²⁺ exceeds that of seawater, and Na⁺-Ca²⁺ and Na⁺-Mg²⁺ appear. There is a smooth step for Mg²⁺ when it reaches maximum value, Na⁺-Mg²⁺ takes place at that time,and K+ is mainly showed as chemical absorption. Mineral disso-lution and precipitation occur with ion exchanges. Calcite reaches saturation when the percent of seawater comes up to 40%. The saturation of gypsum first becomes higher,and then drops when it reaches peak value, and finally rises slowly, but gypsum is in non-saturation state from beginning to the end.