Abstract: An experimental study was conducted to analyze the assembly failure of high lock bolts and nuts.Based on the results of the high-speed camera, the motion and force condition of the nut during the assembly process were analyzed, and the causes of nut failure were furtherly discussed based on fracture morphology. The results show that the nut generates bidirectional bending motion during the assembly process. The bending stress is the main source of load leading to the nut fracture, and the maximum positive bending stress is 594 MPa.The nut fracture mode is bending fatigue fracture.