When the 34th sucker rod was set down into the well, the first tubing of the string was tripped from the mill end, and all the entire well's tubing and the sucker rods dropped into the well, the tubing was tripped three times during the salvage process, and all of them was tripped from the mill end. The specification of failed tubing was Φ73.02 mm × 5.51 mm J55 NU. To analyze the cause of the tubing failure, three groups of samples (failed tubing, new tubing and used tubing) were analyzed by means of mechanical characteristics, thread parameter inspection, microstructure analysis, and physical performance test. The results indicated: (1) the mechanical characteristics of failed tubing and new tubing met the standard requirements; (2) the thread parameters of the new tubing met the standard requirements, and there was no galling after makeup or breakout; (3) the strength of the thread connection after stretching to failure of the two groups of tubing that had not failed met the standard requirements, but the failure position was the mill end of the coupling; (4) the pitch fluctuation of the same axis of the new tubing and the outer thread of the failed tubing was obvious, indicating that the mill end thread of the tubing coupling was insufficient to bear the tensile load and had experienced a large plastic deformation during the tripping process; (5) combined with the three occasions of the mill end tripping of the tubing salvage, it was clear that the failure tubing was caused by a smaller makeup torque at the mill end of the tubing. As a result, the connection strength was relatively low, and the field tensile load surpassed the thread connection strength of the tubing mill end. [ABSTRACT FROM AUTHOR]