Abstract: Flaps are critical lift-enhancing devices during the takeoff and landing of civil transport aircraft and are considered essential structural units （PSE components）. Fatigue life must be taken into account during their structural design. During the development of a certain aircraft model, it was discovered that the outer flap arm did not meet the required fatigue performance. To evaluate the fatigue strength of the flap arm, a finite element model of the flap arm was developed. Finite element simulation and analysis were conducted to identify fatigue-critical locations. The fatigue margin of the flap arm was analyzed using the fatigue life-based DFR method. To address the fatigue issues, a structural optimization scheme for the flap arm was proposed. After optimization, the minimum fatigue margin of the structure improved from −0.57 to 0.11, meeting the target design requirements. The improvements were significant, demonstrating considerable practical engineering and scientific guidance value.