Abstract: This paper analyses the sampling of cracked spiral weld seam from a pipe with optical microscope, scanning electronic microscope, energy dispersive spectrometer, Vickers hardness tester, and metallographic observation and other analysis means, and tests the anti-cracking performance of base material of pipeline with impact tests.The results show that there are obvious defects in pipeline seam, the types of defects contain welding metal cracking and inclusions in welding seam, weld toe cracks and weld cracks in HAZ, induding inclusion both in HAZ and base material.As the hardness of the welding area is higher than that of the HAZ and base material under higher stress, the weld toe crack spreads to the HAZ earlier than the welded metal cracks.The layered inclusion arranged in rolled direction in the HAZ increases the spread of cracks and thus forms layered cracks.which finally cause pipeline cracking to spread to base material along HAZ.