Abstract: In this paper, the analysis on the casing and cladding metal were carried out by macro and micro methods, including the chemical composition analysis, microstructure analysis, fracture metallography and scanning electron microscopy（SEM）. The results showed that the microstructure and hardness of the casing and cladding metal were not uniform. Combined with theoretical analysis, it was concluded that the fracture property of retaining tube was fatigue fracture.The cause for fracture was welding residual stress, which results in the crack initiation under the vibration of fluid. It was suggested that the welding of the retaining sleeve should be avoided.