The bolt of tower crane was suddenly fractured after operating for one year. By analysis the chemical composition，mechanical properties, microstructure, impurity and pattern of fracture, the analysis shows that the grade of impurity exceeds the application criterion, the finish of The bolt face is not good, heat treated technology is irrationality, dehydrogenization treating could meet the technology require, these lead The bolt broken under the alternate load, At the same time, this article puts forward the relevant improving measures.