Abstract: According to Rayleigh-Lamb equation, the dispersion curves of phase and group velocities of 316LMod stainless steel linings were derived out. The main Lamb wave mode of backside defect was drawn out through 2D FFT of echo signal of the sample with artificial defects, then, the Lamb wave testing parameters were confirmed. Through the defect recognition of the sample, the defect equivalent evaluation was given, and then, the quantitative Lamb wave testing of backside defects was realized. Finally, the method was verified by examples of urea reactors testing.