Abstract: The ball socket bearing of pantograph pull rod in a metro failed and fractured during operation. The failure causes of ball socket bearing were analyzed by means of macro analysis, fracture analysis, metallographic examination, hardness test and chemical composition analysis. The results show that the fracture mode of ball socket bearing of pantograph pull rod belonged to high cycle and low stress fatigue fracture. In the service process, the ball socket bearing was subjected to frequent vibration and alternating load. The micro cracks at the corner became the source of fatigue cracks. The cracks propagated continuously under the alternating load, which eventually led to the failure fracture of the ball socket bearing.