In order to realize quantitative detection of the surface-breaking defects by laser ultrasonic technology,the mechanism of the surface acoustic wave and the surface-breaking defects are studied.In this paper,the finite element method is used to study the interaction between the front and rear edge of the surface defects and surface acoustic wave,respectively.Then the influence of the existence of defect width on the front of the defect and the surface acoustic wave is discussed.Finally,the characteristic quantity (oscillation signal) which can quantitatively characterize the surface defect is given by analyzing the rectangular defect interaction with surface acoustic wave.The simulation results show that the oscillation signals (feature points W and E) after RS wave (feature point Q) originate from the oscillations of the transmitted Rayleigh wave at the rear edge of the defect.The arrival time of feature point Q showed a trend of slight increase linearly with the increase of defect depth or width;the arrival time difference of feature points W and E increase linearly with the increase of defect depth,which is independent of the defect width.The depth of the surface defect is quantitatively calculated according to the arrival time of the feature points.The research results will provide a theoretical basis for detecting surface defects.