The method of silt extrusion by dumping riprap has been widely applied in the foundation treatment of coastal high-grade highway. However, the reinforcement mechanism is rarely researched. Elastic-plastic finite element numerical simulation was performed to analyze the stress and deformation behavior of subgrade treated by dumping riprap to extrude silt. The results indicate that the foundation modulus after riprap treatment can be increased effectively, the settlement and horizontal displacement can be decreased and the dissipation of pore water pressure can be accelerated, which suggest that the reinforcement mechanism lies in artificial shell effect and accelerated consolidation effect. The settlement of subgrade decreases with the increase of riprap thickness, but is influenced little by riprap modulus and width. Therefore, effective measurements must be taken to increase the depth of riprap in construction. The quicker of subgrade filling, the bigger of subgrade settlement; and the shorter of preloading period, the bigger of subgrade settlement, therefore the filling speed must be controlled strictly and sufficient preloading period must be assured so as to meet the requirements. Understanding of the reinforcement mechanism, the stress and deformation behavior and the influencing factors is conducive to providing reference and guidance for the design and construction of riprap silt extrusion. 