The mechanism of the laser-induced surface acoustic wave interacting with surface-breaking defects is studied by using the finite element method. The characteristic quantity (oscillation signal) quantifying defect depth is selected from the displacement signals and the influences of the defect depth and the width on the oscillation signals are further analyzed. The source of oscillation signals is clarified according to the displacement field occurred in the interaction between the surface acoustic wave and the rear edge of defects. The numerical results show that the oscillation signal with feature points of A and B originates from the oscillation induced at the rear edge of defects by the transmitted surface wave. At the same temperature, the arrival time difference between feature points A and B increases linearly with the defect depth, but is independent of the defect width. According to the relationship between the arrival time difference and the defect depth, the quantitative calculation of defect depths at different temperatures is realized by combining the relationship between the surface wave speed and the temperature.