The changes in crystal structure of LiFeSi(2)O(6) induced by the phase transition between the high-temperature C2/c and low-temperature P2(1)/c phase are studied using the density functional theory. For both monoclinic phases, the phonon dispersion curves and phonon density of states are calculated. The infrared absorption coefficients are obtained and analyzed in both structural phases of LiFeSi(2)O(6). The soft mode inducing the phase transition is revealed at the Z point of the Brillouin zone of the high-symmetry C2/c phase. The pressure dependence of the soft mode is studied and the mechanism of the structural phase transition in LiFeSi(2)O(6) is discussed.