Abstract: A 500 kV transmission wire fractured during the construction process. The reasons of the fracture were analyzed by macroscopic observation, geometric size detection, chemical composition analysis, mechanical property test, fracture surface macro and micro analysis, simulation test and so on. The results show that the structure size, mechanical property and material of the wire were normal, the fracture was the typical forward tensile fracture, and no defects were found. Through the design of the wire layer-by-layer tensile test and the simulation test of the field mechanical condition, it was found that the wire did not slip and fracture when the tightener was loaded to the rated load, and the whole fracture happened when the load was close to the rated pulling force of the wire. In comparison with the fracture morphology of the specimen and the fractured wire, it was determined that the wire was overloaded in the construction process, layer-by-layer fracture occured at the clamping position of the tightener.