Auxiliary power unit(APU) system is installed in the APU cabin at the rear fuselage of the aircraft. As the designated fire area in the airworthiness review process of civil aircraft, APU cabin needs to undergo strict fire tightness verification to ensure that the fire risk that may occur in the fire area of APU cabin will not spread to other areas of the aircraft, thus affecting the safe operation of APU system and even the whole aircraft. The APU system communicates with the outside atmosphere by the APU inlet system through the APU cabin firewall, and provides fresh air for the normal operation of the APU. The inlet system passing through the firewall puts a great challenge on the fireproof and sealing ability of the APU cabin. Civil aircraft airworthiness certification also puts forward detailed airworthiness requirements for the fire protection of APU inlet system. Starting from the interpretation of APU inlet system fire airworthiness provisions, through a civil aircraft APU inlet system fire airworthiness certification example, the fire performance of the inlet system was studied from the design point of view, and the fire sealing areas that need to be focused on in airworthiness review were tested.