Abstract: Based on field geohydrological investigation,grain size and mineral compositions of the aquifer media collected from the downstream of Dagu River were analyzed.With sand tank laboratory experiment,the change law in hydraulic conductivity of aquifer medium in salt-freshwater displacement process was investigated.The result shows that there are significant changes in hydraulic conductivity of aquifer medium when the saltwater and freshwater displace each other.The reason for changes of hydraulic conductivity is that the migration,flocculation,sedimemt of released clay particles such as illite,kaolinite and chlorite ocur due to salinity changes.Changes in hydraulic conductivity are irregular.In various horizontal distances from water let and various heights,the change law in hydraulic conductivity of aquifer media is obviously different.