Fresh groundwater resources in coastal regions are valuable but deteriorated by seawater intrusion. To prevent deterioration of the fresh groundwater resources, a common approach is to build cut-off walls. However, the construction of cut-off walls may trap a large amount of residual saltwater in the landward aquifer. This study explored the dynamic behavior and the desalination process of the intruding seawater after the construction of a cut-off wall, using a numerical model validated against a laboratory experiment. Field-scale simulations reveal that a fast saltwater repulsion process proceeds within a short period (100-150 days), while the retreat of the residual saltwater wedge is rather slow at the beginning. This is because the desalination process always starts first in the vertical direction. Meanwhile, the saltwater area significantly increases in the downstream of the cut-off wall. The amount of residual saltwater mainly depends on the depth and the hydraulic conductivity of the cut-off wall, while the retreat time is affected by the location of the wall. Although a deeper cut-off wall can enhance the removal of residual saltwater, the desalination effectiveness may notably decrease when the depth exceeds a certain value (25 m in the simulated scenarios). The desalination performance degrades dramatically when the hydraulic conductivity of the wall is greater than a certain threshold (8×10^-7 m/s in the simulated cases). A near-complete removal of residual saltwater can be achieved within three years with optimal designs of the depth, location, and hydraulic conductivity of the cut-off wall.