Abstract: The bolts of coupler yoke fractured in the head of screw near the location of the roots.The reasons for the bolts fracture were analyzed by microcosmic analysis tools. Comprehensive analysis showed that the side of the head of the fractured bolts were likely to be heated locally then forged into the square,and be overheated simultaneously. There were larger difference between microstructure from the side of the head to that of the screw thread. The transition zone of the martensite structure and the widmanstaten structure was formed ahead of the U shape near the fracture,and there existed larger tensile stress than other zone. Since non-performing organization existed,the brittleness of steel increased,cold working performance decreased greatly and microcrack initiated. When the bolts of coupler yoke were too large impact in the course of train operation,the microcrack on bolts splayed and expanded rapidly,then the bolts fractured finally under the combination of tensile stress along axes near the side of head and composition of forces of normal fastening bolts.