Abstract: This paper uses dynamic contact FEM to analyze impact and collision course of dynamic compaction to foundation soil. There is local large deformation in the peripheral foundation soil of the compacting spots. Since the mechanical derivation of infinitesimal strain can’t eliminate effect of rigid displacement, large deformation theory must be used in it. Erosion element is used in calculation model for ensuring the mesh stability of explicit algorithm. The hammer’s contact force and contact displacement are worked out in the course of calculation,and dead weight of hammer is taken into account. The model used can simulate the separating or contacting course between hammer and foundation soil and it is applicable to cases of complex foundation soil structure. At the end of this paper, an engineering example is discussed to test and verify this model.