Abstract: Offshore freshened groundwater (OFG) is a natural freshwater resource located mainly in the continental shelf region, from either onshore coastal aquifers or paleo-channels formed in sea-level lowstands. Conventional seismic methods have certain limitations in identifying salinity changes. Fortunately, the marine CSEM (controlled source electromagnetic method) is sensitive to high-resistivity thin layers, which is beneficial for detection of OFG by analyzing electromagnetic anomalies caused by the contrast of resistivity between the freshwater reservoirs and surrounding sediments. Paleo-channels in Shengsi in the Yangtze River estuary were studied. Based on the hydrogeological and logging data of "Shengsi No. 1 Well", a geoelectric model was established to analyze the marine CSEM responses. Results indicate that the marine CSEM could effectively detect electromagnetic anomalies caused by high-resistivity thin layers, and has good ability to locate underground freshwater reservoirs; its application for the detection of OFG in the Shengsi paleo-channel is feasible.