Abstract: There is obvious electrical difference between salt and fresh water interface, so electrical detection method is a reasonable choice. In this paper, the resistivity sounding method is used to detect the underground salt and fresh water interface in Xianxian County, Cangzhou. First, combined with the comprehensive study of hydrogeological conditions, the physical properties of different lithologic strata in the working area were tested by mini-four-electrode array. According to the geophysical precondition that there are obvious resistivity differences on both sides of the underground salt water and fresh water interface, seven lines were arranged, and inversion and interpretation were carried out to find out the distribution of salt water and fresh water at the depth of 0~250 meters underground in the study area. The study shows that the salt and fresh water boundary in the region is nearly NE-trending. The northwest area of the boundary is about 1/3 of the fresh water area, and the southeast area of the boundary is slightly more than 2/3 of the salt water area. There are three small fresh water areas in the salt water area. The depth of the interface between salt water and fresh water is generally shallow in the west and gradually deepens to the east, and there are four funnel areas with different depression amplitude, different influence areas and characteristics. This study shows that the resistivity sounding method has good effectiveness in groundwater exploration and salt-fresh water interface exploration.