Seawater intrusion is a major issue in coastal areas, and the traditional technology of investigation mainly makes use of drilling for water level observation and sample analysis of groundwater; as drilling data is an information of points, it will cost more if it is extended to be information on the surface. Non-destructive geophysical exploration technology has been used in the investigation of the underground environments at all kinds of contaminated sites in recent years. Seawater intrusion is also a groundwater pollution situation, and "Surface and Space" information can be obtained with a small amount of drilling data. This research will use Electric Resistivity Tomography as a geophysical exploration technology to assess the situation at the interface of seawater intrusion and to explore the dynamic changes of seawater intrusion through long-term monitoring. The test site of this research is situated in the seawater intrusion area along Kinmen Island. The research results showed that the front edge of the seawater intrusion could be detected with Electric Resistivity Tomography, and a delay effect existing between seawater intrusion and tidal action was observed by means of Time-lapse. If a long-term observation system is established in future, it could contribute to the confrontation of seawater intrusion and salinization phenomena.