In the framework of the phenomenological theory of Landau-Ginzburg-Devonshire, the features of the phase transition in a thin ferroelectric film in external electric field are investigated depending on the film thickness and the type of polarization fixed on its surface. It was found that the phase transition mechanism changes in an electric field. In addition to specific diffusing, which differs from diffusing in a bulk material by the appearance of a temperature range with an anomalously high susceptibility near temperature Tc, the phase transition shifts in temperature, and the shift value depends on the film thickness, its surface properties and in a weak fields does not depend on the field strength.