Abstract: Seawater intrusion poses a significant potential threat to coastal underground engineering, and the accurate identification of its preferential flow pathways is crucial for risk prevention and hazard mitigation. Traditional assessment methods relying on observation wells and groundwater sample analysis have inherent limitations, such as discrete spatial distribution, temporal lag, and high costs. Therefore, it is necessary to develop non-invasive, spatially, and temporally continuous efficient detection techniques to enable rapid identification and delineation of seawater intrusion pathways. Electrical resistivity tomography (ERT) presents a clear response to low-resistivity structures such as water-bearing fracture zones and faults, effectively reflecting the electrical property differences of subsurface media, and thus facilitates the rapid and convenient detection of potential seawater intrusion pathways. To improve detection accuracy and inversion reliability, a finite-infinite element coupling method was employed to achieve high-precision numerical simulation of the direct current resistivity method. On this basis, an ERT inversion study for seawater intrusion detection was conducted using the conjugate gradient method. Validation using a layered resistivity model shows that the coupled method significantly improves numerical simulation accuracy, reducing the maximum relative error from 30.7% with the traditional finite element method to 1.7% with the coupled method. Numerical examples and a field case further demonstrate that ERT can effectively delineate low-resistivity structures formed by seawater intrusion pathways, with imaging results showing good agreement with actual hydrogeological data and revealing the spatial distribution characteristics of seawater intrusion. ERT enables rapid and areal hydrogeological investigations related to seawater intrusion, overcoming the limitations of traditional point-based sampling methods. It provides an economical and convenient geophysical exploration approach and technical support for preliminary site selection surveys as well as subsequent monitoring and maintenance of coastal underground engineering, holding significant engineering application value.