Abstract: After about 1000 representative flights, about 60% of the adjusting joints of an aircraft undercarriage were found to have cracked during magnetic particle testing. In the present work, the crack mode and cause of the joints were analyzed by micro and macro observation, structure observation and hardness testing. The results show the failure mode of the joints is fatigue, and the main cause is that the joints have no sufficient safety margin. The fatigue area occupied more than 90% of the facture surface of the joint test specimen, so it can be assumed that the joint has a good cracking tolerance. Base on damage-tolerance design and fracture mechanics, a reverse deduction method for fatigue life was discussed, which can be used to assess the safety and maintenance of the joints.