Investigations into the phonon behavior provide important information on interactions between different excitations in quantum materials.We perform a detailed study of the phonon behavior in the topological insulator Sn-doped Bi1.1Sb0.9Te2S using infrared spectroscopy.We observe two IR-active phonon modes at about 64 and 165 cm-1,which are labeled with α and β,respectively.While the evolution of the β mode with temperature can be well described by the expected anharmonic decay process,the α mode shifts to lower frequencies with decreasing temperature.Such an anomalous softening of the α mode may arise from the charged-phonon effect due to the coupling between this mode and the topological surface states in Sn-doped Bi1.1Sb0.9Te2S.