Abstract: The high strength bolt of 35CrMoA steel for a tower crane fractured during service. The causes of bolt fracture were studied by means of fillet radius under bolt head measurement, chemical composition analysis, mechanical property test, macroscopic examination, fracture analysis and metallographic examination. The results show that the bolt was not tightened in time after loosening during service, causing stress concentration in the local area of the bolt, and forming a fatigue source, resulting in fatigue fracture of the bolt.