Abstract: The small end of variable cross section transition zone of thread positioning pin of 0Cr16Ni6 hot rolled annealed steel fractured during service. The macro observation, scanning electron microscope analysis, chemical composition analysis, hardness test and other comprehensive analysis were carried out. Combined with the fatigue test, torsion test, thermal exposure test, magnetic particle testing, simulation and other related tests, the fracture process and cause of the screw positioning pin were analyzed and verified. The results show that the possibility of material defects and machining original defects could be excluded; the improper assembly process of the screw positioning pin caused the surface scratch of the parts, and the stress concentration occured at the scratch site, which reduced the fatigue strength of the screw positioning pin and eventually led to fatigue fracture.