By embedding settlement monitoring instruments in a typical section of a seawall road widening project in a sea area, the impact of the new embankment during its construction on the settlement of the old seawall was studied. Measured results indicate that the construction of the new embankment has caused significant settlement of the old seawall, with a large difference between the amounts of settlement at its left and right road shoulders, which leads to cracking. Simulation analysis using Plaxis finite element software shows that the horizontal displacements in opposite directions, with maximum values of 0. 5 m and 0. 8 m, occur under the old seawall and new embankment, respectively, and that the simulated results agree with the measured data. Thus, appropriate corrective measures should be taken in response to differential settlements during the design of the seawall road widening project.