Abstract: The multi-modes and dispersive effect of ultrasonic guided wave leads to the spreading of the wave-packet propagated in a structure. The signal of ultrasonic guided wave propagated in a pipe at 400 kHz are studied. Results show that time-frequency analysis method can be used to describe the energy distribution of guided wave signal , give the main modes information and show the trend of group velocity changes. From these, the corresponding relation between the signal in time space and group velocity dispersion curves can be established. A reassignment method for nonstationary time-frequency analysis creates a modified version of a representation by moving its values away from where they are computed, so as to produce a better localization of the signal components and improve the readability of the representation in the study.