Abstract: In this paper, a method scanning the workpiece time after time in the frequency range is intraduced.This method is simple and effective for the study on the mechanism of VSR.The experiment has been done on a beam type workpiece.Based on the abtained scanning curves,the law about the dynamics of VSR process has been worked out shearing stresstakes an important part in the VSR.A qualitative criterion which can evaluate the VSR result on-the-spot has been put forward.