The temperature dependence of the phonon frequencies and effective charges in the superionic material Ag3SI is reported. It was found that the effective change decreases with temperature T in the gamma-phase, jumps at the gamma-beta phase transition point T=157 K, remains almost insensitive to T until immediately above the beta-alpha phase transition at T=519 K, and increases rapidly with T in the alpha-phase. The behavior has been explained based on a model in which the basic process is the local fluctuation of the chemical bond.