Abstract: The damage detection of liquid layer loaded thin-plate structure is a key issue in structural nondestructive testing and health monitoring. In order to find a suitable damage detection signal of liquid layer loaded thin-plate structure and analyze the response of the damage defect to the detection signal, this paper solves the dispersion equation and calculates the dispersion curves of the 4 mm aluminum plate loaded by double sided semi-infinite water layers. Additionally, the effects of the position, angle and size of the defect on the acoustic detection signal in aluminum plate are investigated by using finite element simulation methods. The results show that the attenuation coefficient of the S0 mode leaky Lamb wave at the central frequency less than 100 kHz is close to zero, which is suitable for long-distance damage detection. When the presence of defects causes the asymmetry of the plate structure, the acoustic signal will have a significant mode transition at the defect. And, as the distance from the defect location to the center of the plate increases, the transmission coefficient of the converted A0 mode signal decreases, with the increase of the defect length and the angle between defect and medium layer, the transmission coefficient of the A0 mode signal increases first and then decreases; and with the increase of the defect width, the transmission coefficient of the A0 mode signal increases.