By means of chemical analysis,mechanical properties test,microstructure analysis and particularly combined with the joint force and deformation analysis, longitudinal cracking reasons of box joint of 1397 mm double shoulder drill pipe were studied. The results show that the reasons for cracks are: over torque operations and unreasonable subshoulder gap settings, which result in severe stress concentration near the subshoulder surface and the 1st2nd thread. When releasing stuck pipe, dramatic friction occurs between screwed threads, resulting in white layer because of heat and then initial crack occurs. Also there is a certain H2S corrosion in the environment,and materials transverse impact energy is less than the longitudinal impact energy, eventually cracking occurs in the longitudinal threads. Improving the structure of the joint by changing the joint taper from 1∶6 to 1∶12 or 1∶16 will prevent the occurrence of such incidents.