Abstract: Square drilling pipe was mainly used for torque transmission and drill string weight bear, so it was necessary to carry out ultrasonic testing for butt weld. The ultrasonic testing methods, steps, defect positioning methods and the quality determination were introduced. At the same time, the formation and waveform characteristics of some typical weld defects(lack of penetration, cracks and pores) were introduced. Years of practice had proved that the use of ultrasonic testing method was practicable for butt weld of square drilling pipe.