Integrally T-stiffened plates are widely used in aerospace, marine and other engineering structures. When elastic guided wave method is used to detect the damage of such structures, it is necessary to well understand the wave propagation behavior in these structures to further mature the guided wave SHM method. In this paper, the interaction between the fundamental anti-symmetric guided Lamb mode (A0) and T-stiffener in an integrally stiffened thin plate is studied by finite element numerical simulation. The propagation behavior of A0 mode in stiffened plate is actuated using a piezoelectric wafer active sensor. The characteristics of wave propagation caused by the presence of stiffener are analyzed in time domain using response signals and displacement snapshots. The results show that the existence of stiffeners causes obvious mode conversion. With the change of excitation frequency, the reflection and transmission behaviors of guided waves in A0 and S0 modes change obviously.