Abstract: By means of macroscopic inspection, chemical composition analysis, mechanical property test, metallographic examination, energy spectrum analysis and so on, the fracture causes of 10.9 high strength bolt head were analyzed. The results show that in the forming process of bolt head,folding micro cracks formed on the R angle area of head,which became the early crack source. During working time, the bolt head subjected to alternating work stress for a long time, thereby furthering the expansion of micro cracks, and eventually caused the high cycle fatigue fracture. Finally, the relevant preventive measures were put forward.