Abstract: The steel core inside the strain clamp of ultra-high voltage direct current transmission line fractured. As for the fractured steel core, chemical composition analysis, mechanical properties testing, fracture analysis and metallographic examination were carried out. In addition, the crimping process was analyzed and studied by the methods of appearance size inspection and crimping trace comparison of the failed strain clamp, and crimping verification experiment and simulation calculation analysis were carried out on the judgment results. The results show that the fracture of the steel core in the conductor is ductile fracture. During the crimping process, due to the jump crimping sequence of “conductor side—steel anchor side—conductor side”, stress concentration occurred to the inner steel core of the strain clamp during the crimping process. The internal steel core fractured due to the external forces such as wind load and ice covering during the service of the line. It is suggested that the crimping process should be strictly implemented during crimping large cross-section conductors, and the crimping process of “crimping sequentially from steel anchor side to conductor side” should be adopted.