Abstract: A flow booster coupling bracket joint bolt fractured during operation. The reason for fracture of the bolt was investigated by fracture morphology observation, chemical composition analysis, hardness testing, microstructure analysis, intensity check and finite element simulation. The results show that the bolt failed in fatigue fracture. The stress concentration at the root of the first thread at the joint of the bolt and nut was obvious. Under the alternating tension loading, cracks initiated at the spot and propagated until the bolt fractured. The low content of nickel and molybdenum in the bolt material, the high hardness, the defects such as holes and inclusions in the structure, the obvious stress concentration at the root of the thread and the large alternating stress amplitude were the main factors leading to the fatigue fracture of the bolt.