Abstract: Analyzed in the essay are the causes for the cracking of the N80-1 tubing during the hydrostatic test.It is revealed via checking that the fracture of the failure tubing is a kind of typical brittle fracture,and that scratch defect is found on the inner surface of the pipe section in front of the fracture,where the stress is concentrated,and when pressed,cracks are initiated at the bottom of the scratch defect,which is just the incentive for the said failure;and furthermore the serious banded structure as existing in the material gets the transversal mechanical properties weakened,and leads to poor crack-resistance,which is regarded as the major reason for the above mentioned cracking.