Trailing edge flaps are critical highlift devices for modern airliner and mechanisms are the most important structural supports. Based on structures and damage tolerance, the requirements and usage scenarios of failsafe design for flap mechanisms are provided. Based on the crack growth method and the principle of independent crack growth of primary and secondary structures, the crack growth curve, inspection threshold and repeated inspection interval of different types of failsafe structures are determined. According to the connection characteristics of hinged flap and sliding track flap, the classification method of the sameplaced structure and the differentplaced structure are proposed from the relative positions of the primary structure and the secondary structure, and the classification method of the plate structure and hinge structure are proposed based on the structure characteristics. The minimum installation clearance design methodology is given, which is of great significance to the development of civil aircraft.