A propulsion system for an aircraft, including a dual-flow turbojet and a mounting structure for mounting this turbojet on the wing surface or on the fuselage of an aircraft. The mounting structure includes an aft aerodynamic fairing including a thermal protection floor to protect the mounting structure from the heat of a primary airstream channelled by an exhaust nozzle of the turbojet, as well as an air inlet provided in a longitudinal aerodynamic wall washed by a secondary airstream of the turbojet and delimiting together with an other similar wall a cavity isolated from the secondary airstream for extracting a cooling airstream from the secondary airstream, and air circulation means fed by the air inlet and having at least one outlet aperture emerging in a space between the thermal protection floor and the exhaust nozzle.