Abstract：With the sandy aquifer lying in the seawater intrusion area of the Dagu River in Qingdao as the study object, the authors collected groundwater, seawater and the media to evaluate their composition and properties. Through seepage simulation of the process of seawater and fresh-water, the authors determined the regularity of the permeability and the main ions in the course of seawater and fresh-water displacement. Numerical simulation was also employed to study water sensitivity under the influence of ion exchange. The results indicate that, in the process of seawater displacing freshwater, ion exchanges of Na+-Ca2+, Na+-Mg2+ and Na+-K+ take place, with the ion exchange speed of Na+-Ca2+ and Na+-K+ fairly quick but that of Na+-Mg2+ relatively slow. During the course of freshwater displacing seawater, Ca2+ and Mg2+ in solution can replace Na+ and K+ in the media, with Ca2+- Na+ playing the dominant role. The kinetic parameters and the turning point of the breakthrough curve verify the water sensitivity phenomenon in the seawater and fresh-water transitional zone, Multi-component ion exchange leads to the phenomenon of water sensitivity.