Abstract: Rotary drilling machines are a kind of foundation drilling engineering machinery, and their drilling masts are generally fastened with high-strength bolts. After a rotary drilling machine was used for 1000 to 2000 hours, the high-strength connection bolts of the drilling mast fractured for several times. In order to find out the fracture cause of the bolts, macro and micro inspection of the fracture of the fractured bolts was carried out, and tensile and impact properties testing, hardness distribution testing of cross section, thread bottom arc inspection, as well as analysis of surface decarburization and metallographic structure of surface and core were performed on the same batch of new bolts. The results show that the fracture mode of the bolts is fatigue fracture at thread. The mechanical properties of bolts meet the standard requirements, but there are sharp shapes, cracks and other defects at the thread. The form of the defects is mainly because there was folding in the process of wire rolling and the rolling plate was not replaced regularly. Under alternating load, cracks were inclined to initiate from the thread defects, eventually leading to the failure of the bolts.