To improve the strength of the drill pipe in the large-size hole， the ?168.3mm S135 drill pipe was used for the first time in the extra-deep well， which effectively guarantees the safety of the drill pipe. However， various failure forms are still found in the drill pipe inspection. To study the failure causes of the ?168.3mm S135 drill pipe in the large-size hole， the failure causes were identified through tracing the life cycle of the drill pipe， statistical analysis， drill string mechanics analysis and experimental study. The research shows that the main failure types of ?168.3mm drill pipe in large hole are thread wear， drill pipe bending， drill pipe corrosion and fatigue failure. Thread wear is mainly caused by the long use time of drill pipe and repeated make-and-break operations. Besides， the conventional API thread structure also needs to be optimized to improve the thread resistance to wear. The bending of drill pipe is caused by the plastic deformation of drill pipe which is subjected to large bending stress for a long time under large tensile load. The corrosion of drill pipe is caused by the increase in corrosion rate in the environment of air-water-Cl^- due to the residual drilling fluid on the surface of drill pipe. The fatigue failure of drill pipe is caused by the obvious stress concentration of drill pipe near drill collar and the large bending deformation of drill pipe in large hole. Based on the analysis of the failure causes of drill pipe， the measures to prevent the failure of ?168.3mm drill pipe in large hole are put forward. The research results can provide guidance for the safe application of S135 steel ?168.3mm drill pipe in extra-deep Wells with large size boreholes.