In a propulsion unit for an aircraft, including a nacelle and a turbojet engine supported by a pylon, a nacelle includes an outer shroud, a fixed internal structure defining, with the outer shroud, an annular space in which a cold air stream can circulate, and a sealing ring positioned between the fixed internal structure and the pylon. The sealing ring is used to separate a cold pylon zone from a hot engine zone. The propulsion unit includes a ventilation device allowing a portion of cold air of the cold air stream to be deviated to the sealing ring in order to ventilate the sealing ring.