Based on dynamic contact force and FEM, the contact – separation process of collision was investigated. Compared with other methods, the dynamic contact method needn’t modify the general stiffness matrix, and increase any additional iterative computation. So there should be less computation and high efficiency. By building simulation model LECEI(the technique of loading edge cracks by edge impact), the propagation of branching cracks were simulated. Compared with existing research results, the method can be suitable to the impact model such as LECEI.