Abstract: As a common case that is encountered with when manufacturing the 45MnCrMo drill collar pipe,lots of cracks are developed on the inner surface of the pipe,which negatively affect the pipe quality and the yield. Accordingly,relevant analysis and study are conducted to the pipe to identify the characteristics and formation causes of the said inner surface defect by means of such methods as LM,SEM,EDS,thermal simulator and macroscopic hot acid etching process etc. The results of such tests reveal the true story about formation of the defect. It says that the inner surface cracks of the 45MnCrMo drill collar pipe are developed due to the overheat-caused plasticity reduction and grain boundary weakening in local areas of the pierced shell as induced by higher temperature rise of the metal of the pipe inner surface and the piercing plug,which is jointly caused by such complex stresses as the friction,and the torsional deformation,etc. in the piercing process,and that the crack formation has nothing to do with the metallurgical quality of the blank. As a result,countermeasures are put forward to improve the inner surface quality of the drill collar pipe.