Abstract: A batch of 40Cr steel screw bolt broke continuously early during the fatigue test, failing to meet the required times of test. In order to explore the cause of this batch of fastening shaft fracture, the fracture component analysis, hardness detection, macro fracture analysis and scanning electron microscope, metallographic tissue detection and thread contour measurement, the results show that the thread of fastening shaft bottom transition arc radius R≤0.07mm, and the alignment more than 6 level unqualified, resulting in the fastening shaft fatigue test thread root stress concentration formed subtle cracks and gradually expand and fracture. Through analysis and discussion, the improvement measures are formulated and the production process of the fastening shaft is strictly controlled, and finally the mechanical properties of the fastening shaft meet the technical requirements.