Abstract: The fracture occurred to all the 36 high strength bolts connecting the nether frame with the slewing bearing of the digging machine during use. By means of macro examination, chemical composition analysis, mechanical property testing, metallographic examination and so on, the fracture reasons of high strength bolts were investigated and analyzed combined with the factor of bolt assembly and working condition of the digging machine. The results show that: the fracture mechanism of the bolts was fatigue fracture; the fracture reason was that the assembly of the bolts was unqualified and it made the bolts slightly loose during use; under the action of the shear force of the digging machine, the fatigue cracks generated in the loose position of the bolts during working process, and finally the bolts fractured.