Abstract: By means of macroscopic examination,chemical composition analysis,metallographic examination and mechanical properties testing,combined with the use condition and the applied load,the piercing reason of S135 drill pipe was analyzed. The results showed that the failured drill pipe was in the neutral point position of the string and bore the alternating stress and bending stress,which resulted in the fatigue crack initiation and expansion quickly in the bottom of the corrosion pits ,which with stress concentration in the outer surface of drill pipe.