The reflection coefficients of surface waves excited by pulsed laser at different normalized depths in the near field of surface defects were studied using finite element method, and the signal enhancement phenomenon of surface waves at the near field of defects was verified. At the same time, by analyzing the variation of reflection coefficient with distance, the concept of the radius of the reflection signal enhancement region was established, and it was found that the radius of the signal enhancement region is related to the depth of surface defects. A formula for measuring the depth of surface defects was fitted; We also analyzed the influence of rectangular defect width on depth measurement and corrected the formula for measuring surface defect depth. In addition, based on the phenomenon of reflection coefficient changing with defect depth, the relationship between normalized depth and surface wave length was analyzed. The research results indicate that there is a shortwave limitation when using reflection coefficient to measure defect depth, which means that when the surface wave length is less than half of the defect depth, the reflection coefficient is independent of the defect depth, and at this point, the reflection coefficient is 0.4.