Abstract: The titanium alloy high lock bolts of an aircraft fractured during assembly. The causes of bolts fracture were analyzed by means of macro analysis, micro analysis, metallographic examination, mechanical property recheck and assembly site investigation. The results show that the bolts were an overload fracture caused by single side bending stress, which has nothing to do with the bolts manufacturing quality. The operation space of the bolts assembly area was narrow, and the rotation plane of the wrench was prone to tilt during installation, thus formed bending stress on the bolt. It is suggested to improve the assembly tool.