Abstract: The causes for the deformation failure of the coupling shoulder of the Φ88.9 mm×6.45 mm 110S tubing is analyzed by means of macro-morphology inspection, material performance test, micro-morphology analysis of the sealing surface and the shoulder surface, and over-torsion test. And the working torque of the failed tube body is calculated by a mathematical model. The results show that the failure accident is due to the excessive working torque applied to the tubing during the stuck-releasing and string-releasing operations, which makes the shoulder surface of the coupling yielded by over torsion. In addition, the down hole temperature of the failed tubing coupling is above 100℃, which in turn reduces the yield strength of the tubing coupling material. At the same time, the complicated down hole stress condition of the tubing and the possible spiral buckling further aggravate the yield of the coupling shoulder surface, thus inevitably leading to the deformation failure of the coupling shoulder.