A full-scale model test was conducted on the low silt soaking subgrade of the alluvial flatlands flooded by the Yellow River. The test mainly studied the mechanical characteristics and vertical deformation of the silt subgrade under grade cyclic loading in different water immersion conditions. The test demonstrates that the silt subgrade has intense capillarity and water sensitive characteristics; the additional stress is large in the roadbed of the new and soaking silt subgrade, and it decreases sharply as the depth increases; the phenomenon of strength degradation is obvious in the soaking subgrade; the elastic and plastic deformation is large under overloading force and the deformation increases as the water immersion level ascending; The plastic deformation of the subgrade is easy to accumulate in water-saturated areas, where large plastic deformation still occurs in low stress level.