Abstract: Drill collar of a gas well in Sichuan-Chongqing fractured. The causes of fracture on drill collar were analyzed by means of macro analysis, scanning electron microscope analysis, metallographic examination, chemical composition analysis and mechanical property test, combined with field service conditions. The results show that the fracture type of drill collar was corrosion fatigue fracture. The main reason was that the sealing of the main shoulder surface of drill collar was poor, which caused the drilling fluid to erode the shoulder surface and the root of the external thread to form corrosion pits. During the rotary drilling of the drill, the fatigue crack initiated and grew rapidly under the combined action of cycle stress and corrosion, and finally led to the fracture of drill collar.