Abstract: The anti fall T-shaped guide rail of a 1 000 kV transmission line tower fell off. The reasons for the detachment of the guide rail were analyzed using macroscopic observation, geometric dimension measurement, metallographic examination, scanning electron microscopy and energy spectrum analysis, mechanical property testing and chemical composition analysis. The results show that the connecting plate material used did not match the design, and there were a large number of tertiary carbides and severe non-metallic inclusions in the structure of the connecting plate material, resulted in a decrease in the strength of the connecting plate, an increase in brittleness, and insufficient fatigue resistance. At the same time, the punching process for the screw holes on the connecting plate was improper, resulted in excess holes and the formation of cracks at the edges of the bolt holes. During operation, fatigue cracking occurred at the edge of the bolt hole, causd the connecting plate to completely break along the screw hole, ultimately resulted in the detachment of the guide rail.