52 cracks of different length appeared around R corner of counterweight angles were checked in 30 service fighter aircrafts, which affected the completion of the flight training seriously. Based on the parts inspection, strength check and stress of broken components, combined with the results of failure fracture about physicochemical analysis, analyzed cracks on the aileron angle. This paper concluded that the main cause of cracks which is the fatigue source cause of the structure details of the design defects and details of manufacturing defects, and proposed the strengthening scheme about this kind of cracks.