Abstract: To solve the problem that the depth of surface and near-surface defects(d＜2λ) can not be measured. Through the theoretical calculation, the value of the difference amplitude between the crack defects of artificial simulation at different distances with a certain length and the edge of workpiece was obtained, and the later one could be considered as infinitely length and infinitely depth. Comparisons were made with the actual testing data after linear fitting it. The result showed that the theoretical value was nearly the same with that obtained from the experiment. So it could make the corresponding equivalent depth analysis with DGS curves which were drawn by theoretical calculation for the surface and near surface defects. It filled a blank of this field.