The invention relates to a method for preparing fluoropropenes of formula (I) CF3CF═CHR, where R is a hydrogen or a fluorine atom from at least one compound of formula (Ia) CF3CF═CFR, where R has the same meaning as in formula (I), said method including the following steps: (i) hydrogenating at least one compound of formula (Ia) in an adiabatic reactor in the presence of a catalyst with a superstoichiometric amount of hydrogen so as to produce a hydrofluoropropane; (ii) partially condensing the flow from the adiabatic reactor of step (i) so as to produce a gaseous phase fraction, including unreacted hydrogen and a portion of the formed hydrofluoropropane, which is recirculated to step (i), and a liquid phase fraction including the residue of the hydrofluoropropane; (iii) dehydrofluorinating hydrofluoropropane from the liquid fraction of step (ii) using potassium hydroxide in an aqueous reaction medium contained in an agitated reactor so as to produce the fluoropropene of formula (I); and (iv) purifying the fluoropropene obtained in step (iii).