A physical model test is conducted to study the hydraulic gradient and soil plug development during the suction penetration of bucket foundation in the silts. Furthermore, a FEM simulation is carried out by changing the geotechnical parameters and constitutive model to deeply study the suction penetration mechanism. Through the comprehensive analysis, the results indicate that the seepages inside and outside the bucket foundation are the direct reasons for the formation of soil plugs, and that there are the high hydraulic gradient and the safety mechanism preventing soil flow failure for the silts in the bucket foundation. The seepage changes not only the permeability of soil body but also the physical and mechanical properties, and enhances the rheological property and time effect. Thus, the soil plug failure is not seepage failure, not unloading expansion yet, is a complex deformation failure mode under the protection of safety mechanism of bucket foundation.