In order to reveal the settlement mechanism of fine sand subgrade in coastal area, on the basis of a great deal of field investigations and literature research, the fine sand subgrade accumulated settlement based on silting effect was investigated through the humidity analysis, the repeated threeaxis test and the consolidation deformation test. It is pointed out that the fine sand subgrade settlement in coastal area is mainly composed of three parts: permanent deformation, consolidation deformation and sand loss due to fine sand seepagemigrationsilting phenomenon. During the construction period, the rainfall intensity conditions of large rainfall infiltration water causes fine sand migrationsilting phenomenon and makes sand core for a long time in the state of high water content. During the operation, the silting and wet sand core appears excessive permanent deformation in the heavy load traffic. The silting and wet sand core appears consolidation deformation under the effect of soil pressure for a long time. Finally, fine sand subgrade settlement appears.