The low-frequency Raman spectra of ferroelastic Sb5O7I crystals have been measured for temperatures ranging from 295 K to well above the phase transition at Tc=481 K. A strongly damped soft mode which is present only in the ferroelastic phase and quasielastic scattering from a central line, whose intensity peaks around Tc, are observed. A symmetry analysis shows that the instability of the hexagonal high-temperature phase may be explained by assuming a triply degenerate soft mode belonging to the star of the zone-boundary M point in the Brillouin zone. An expression for the free energy consistent with symmetry is constructed and the relation between the spontaneous elastic strain and the soft-mode amplitudes are determined. Applying this phenomenological theory it is shown that the intensities of several Raman lines, which decrease with rising temperature and vanish above Tc, are a measure of the temperature dependence of the order parameter. The experimental results indicate that the phase transition is weakly discontinuous.