It is discovered that, if material has been applied with super-low impact stress that for certain times, macroscopic plastic deformation may be happen. The stress can be far below the static yield limit of material. The strain rate will decrease with impact times and distance from the surface. For exploring the micro-mechanism of the new deformation phenomena, the paper presents the theory of dislocation slip based on the principle of superposed dislocation vibration and the microscopic dislocation slip model under low-stress repeated impact. Analysis showed that the stress threshold is related to not only material damping, but also the frequency of repeated impact. Lower material damping and higher frequency of repeated impact are favorable to dislocation slip and, consequently, favorable to macroscopic plastic accumulation. Stress threshold being closely related to impact frequency is an important feature of repeated-impact plastic accumulation.