Abstract: The relationship between wave-packet duration of ultrasonic longitudinal axisymmetric guided waves with lower order and Inner-Radius-Thickness Ratio (IRTR) of viscous liquid-filled pipes are simulation analyzed. In the condition of take into account the attenuations, at various propagation distances, various frequency-thickness products (fd) and various cycle numbers of the input pulse signal, the dispersion characteristics of guided waves are also analyzed using pulse echo method. The results shown that for a certain material and fd of testing, the dispersion characteristics of guided waves in viscous liquid-filled pipes are only related to IRTR. The cut-off fd of higher order modes will reduces with the increase of IRTR. When using L(0,1) mode to detect the defect of viscous liquid-filled pipes with IRTR at about 4, it has a lower sensitivity. While using L(0,2) mode, it has a lower sensitivity at about 9.2 of IRTR. For viscous liquid-filled pipes of different IRTR, one should select different modes, and chooses different cycle numbers and different fd.