Abstract: Ultrasonic Lamb wave is widely used in nondestructive testing of the plate material. In this paper, the theoretical model of Lamb wave propagating in the large TC4 titanium alloy sheet was derived using the semi-analytical finite element method to solve the dispersion equation of guided waves in complicated medium, followed by the analysis of the characteristics of guided wave and the test of the artificial defects in titanium alloy sheet using S0 mode. The liner scan was performed using S0 mode guided wave for the defects of titanium alloy sheet which were of 5% thickness and 10% thickness in the experiment. The result showed there was strong echo signal where artificial defects existed. So Lamb wave can be used in nondestructive testing of defects in large titanium plates with the advantages of long propagation distance and large detection range.