Abstract: Tension clamp is one of the key connecting parts of transmission lines,and the crimping quality and operational reliability directly affect the stable operation of transmission lines.Through appearance and size inspection,mechanical properties testing,X-ray imaging inspection,disassembly analysis,scanning electron microscopy analysis,energy spectrum analysis,the paper analyzes the reasons of clamp fracture of a ±660 kV DC transmission line with large cross-section conductor.The results show that there is no relative sliding between the aluminum stranded wire and steel anchor,and the main reasons of the steel core fracture are the crimping technology fails to meet the requirements of standards.The reserved gap between steel anchor end face and the aluminum strand end face is insufficient,which makes the steel anchor end face and the aluminum strand end face squeeze mutually,and the steel core is subject to reverse acting force therefrom and eventually ends up with fracture due to the exceeding stress.