The invention discloses a method for preparing pentafluoride ethyl perfluoro isopropyl ketone through gas phases. The method is characterized in that under the existence of addition catalysts, hexafluoropropylene and hexafluoropropylene oxide or pentafluoropropionyl fluoride take gas phase addition reaction to obtain the pentafluoride ethyl perfluoro isopropyl ketone, or tetrafluoroethylene and heptafluorobutyl fluoride take gas phase addition reaction to obtain the pentafluoride ethyl perfluoro isopropyl ketone. The method has the advantages that the reaction route is short; in addition, rawmaterials such as the hexafluoropropylene, the tetrafluoroethylene, the hexafluoropropylene oxide and carbonyl fluoride can be easily obtained; the cost is low; the yield of the pentafluoride ethyl perfluoro isopropyl ketone is high; the continuous large-scale industrial production can be realized through the route.