The well-known description of hydrodynamic instability of planar interfacial boundary at liquid combustion is applied to analysis of stability of a stationary analytical solution for the shape of the phase interface during propagation of self-sustaining evaporation front along a flat heater in superheated liquid. The applicability of the criterion of hydrodynamic instability of planar interphacial surface to the problem with a curved interface is considered. The dependences of the boundary of the perturbed evaporation surface and the range of wavelengths of unstable linear perturbations on the main physical parameters are obtained. It is shown that under certain conditions, no interfacial instability appears in the front part of the vapor cavity. The possibility of change in the mode of propagation of the evaporation front due to instability to azimuthal disturbances is analyzed.