Abstract: Ultrasonic Lamb waves have great potential for structural health monitoring of metal monolithic wall panels, however, stiffeners contained in the wall panels complicate the propagation characteristics of the Lamb waves and limit the application of ultrasonic Lamb waves. In this study, the transmission characteristics of Lamb waves in the typical T-type of stiffener plate were calculated using 2D-FFT method. The experimental and simulation results are in strong agreement, both indicating that the stiffener structure can be considered as a comb filter. The variation rule of the comb filtering effect with the length of the bending on the T-type stiffener was revealed. Furthermore, the ultrasonic field information inside the stiffened plate was tested, and the generation mechanism of the comb filtering effect in the reinforced plate was explained according to the standing wave field theory. This study provides a basis for the reasonable frequency selection of Lamb waves for detecting the stiffened plate structure, expanding the effective detection of defects by Lamb waves, and significantly facilitate the application of Lamb waves in monitoring the health of integrated wall plate structures.