  In this work, we investigate the quasinormal modes of a thick brane system.
Considering the transverse-traceless tensor perturbation of the brane metric,
we obtain the Schr\"odinger-like equation of the Kaluza-Klein modes of the
tensor perturbation. Then we use the Wentzel-Kramers-Brillouin approximation
and the asymptotic iteration method to solve this Schr\"odinger-like equation.
We also study the numeric evolution of an initial wave packet against the thick
brane. The results show that there is a set of discrete quasinormal modes in
the thick brane model. These quasinormal modes appear as the decaying massive
gravitons for a brane observer. They are characteristic modes of the thick
brane and can reflect the structure of the thick brane.
