Abstract: The helical fitting joint of aluminum conductors steel reinforced of a transmission line fractured during operation. The fracture causes of the joint were analyzed by means of macro analysis, fracture analysis and mechanical property test. The results show that the fracture position of the helical fitting joint was in the annular cavity formed between the outer strands of the helical fittings and the steel core of the conductor. During the operation of the line, the outer strands absorbed the vibration energy of the conductor, and fatigue fracture occurred after long-term vibration. After the outer strands fractured, the electric resistance of this position increased sharply and the temperature increased abnormally, which made the steel wire strength decreased significantly, and finally led to the joint tensile fracture.