Abstract: Addressing the problem of frequent occurrence of nonconformance of the Φ1 219 mm×22 mm largesized， heavy-wall X80 SAWH pipe to the guide bending test as identified during the trial production process，investigation into the causes for cracking of the guide bending test specimen is conducted by means of chemical composition analysis，mechanical property testing，and observation of microstructure and rupture morphology of the specimen，etc.As a result，it is found that the said specimen’s cracking is caused by the circumstances as follows.There exists obvious softening of the HAZ of the pipe’s weld joint.A lot of welding defects like slag inclusions and porosities as staying in the rupture area induce the crack source of the rupture.And the bending plastic deformation leads to remarkable stress concentration at the interface between the hard and brittle M/A island in the coarse grain zone near the weld bond and the lath ferrite matrix.Accordingly，relevant suggestions are put forward.