Far-infrared technique, coherent and incoherent neutron scattering have been used to study the lattice dynamics of AgCrS2 at different temperatures. The low frequency TO mode shows a pronounced softening between 10 K and 700 K. The data are analyzed with a rigid-ion model and it is shown that the pattern of eigenvectors of the low energy modes (ν < 45cm-1) give basic information for the diffusion process of the Ag ions