We have performed ultrafast time-resolved spectroscopy of transient absorption for a thin film of Bi3Se2Te, in which recent study result implies the existence of topological surface state. The observed relaxation signal was attributed to the band filling and Auger recombination processes based on the pump power dependence of the signal. After the band filling, bulk-to-surface scattering is thought to proceed being mediated by phonon emission. The dynamics of the phonon was observed from the dumped oscillation appeared on the relaxation curve of the observed signal. The phase of the oscillation indicates that the phonon mode has the period of 519±3 fs in the electronic excited state.