Lattice dynamics calculations for the Cs₂CdBr₄ crystal in the high-temperature orthorhombic phase (Pnma) and in the monoclinic phase (P2₁/n11) at ~195 K were carried out. The model was based on the atom-atom potential function which comprised the long-range Coulombic, short-range and covalent interactions. Comparison of the calculated phonon frequencies with the experimental ones obtained from far-infrared reflectivity and Raman scattering measurements gives a reasonable agreement in most cases. A possible origin of the incommensurate phase transition was discussed on the basis of a coupling between low-lying ₂ phonon branches. Using group-theory analysis and the calculated phonon dispersion branches, it is shown that the P2₁/n11-P phase transition is very probably induced by the condensation of a phonon mode at the Brillouin zone boundary.