Abstract: The 17-4PH stainless steel concentric measuring and adjusting working barrel of downhole tool for water injection well in an offshore oilfield was fractured. The causes of the fracture of the working barrel were comprehensively analyzed by means of macroscopic observation, scanning electron microscope and energy spectrum analysis, chemical composition analysis, mechanical property test, metallographic examination and other methods, combined with the characteristics of working conditions. The results show that the failure mode of 17-4PH stainless steel concentric measuring and adjusting barrel was brittle fatigue fracture. In the process of use, micro cracks first appeared at the root of the thread at the machining tool mark of the barrel, and then under the action of load, the cracks further expanded circumferentially until the working barrel breaked.