Abstract: The connecting bolt of a 35Cr steel gland broke during operation. The cause of bolt fracture was analyzed using methods such as chemical composition analysis, macroscopic observation, metallographic examination, scanning electron microscopy and energy spectrum analysis, hardness testing, and finite element analysis. The results show that there was significant stress concentration in the local area of the bolt, and there was a phosphorus aggregation layer at the bottom of the thread. During the operation of the embedding machine, the bolt was subjected to alternating stress, caused microcracks appeared in the phosphorus aggregation layer at the bottom of the thread, ultimately leading to fatigue fracture of the bolt.