Abstract: In view of the worsening ecological environment in bohai Rim caused by seawater intrusion, 24 sections were set up in Caofeidian and Luannan county of Tangshan, Hebei province. The resistivity differences across rock formations are measureed using the Controllable Source Audio-frequency Magnetotelluric Method (CSAMT). Based on the analysis of the variation law of profile lithology, porosity, water saturation and salinity, the variation law of saltwater body in profile is studied, and combined with the results of hydrogeological survey, the seawater invasion boundary in this area is demarcated successfully. The results show that CSAMT can be used as a method to detect seawater invasion and provide technical support for environmental impact assessment and disaster management.