Abstract: Cracks in hanging welds were found during the maintenance of a traction motor. The failure welds were comprehensively analyzed and characterized by means of appearance and macroscopic morphology detection, scanning electron microscopy observation, structural strength simulation analysis, stress testing and other technical means. The causes of cracks were clarified by combining welding technology and operating environment. The results show that the cracks have certain fatigue propagation characteristics and belong to fatigue failure.The crack originates from the maximum stress position of the motor structure, and the manual welding seam is not smooth, which intensifies the stress concentration in this position.The crack is the result of stress concentration and operation load.When robot welding is used, the weld joint is smooth, and it improves stress distribution and level, which is beneficial to improve the fatigue performance of weld.