Abstract: The critical stresses of welding cold crack for Nb-Mo X80 pipeline steel was obtained by implant tests,it was found that its cold cracking susceptibility was low.The thermal cycle parameters,maximum hardness in HAZ and the original diffusible hydrogen content in deposited metal and diffusible hy-drogen content in deposited metal at 50℃ in different welding processes were measured,and the diffusible hydrogen content in deposited metal at 50℃ was calculated.The cold crack criterion equations of Nb-Mo X80 pipeline steel were established and ana-lyzed.The results show that the cold crack resistance of Nb-Mo X80 pipeline steel increases with the increase of maximum hard-ness in HAZ at the low diffusible hydrogen content,while it de-creases with the increase of maximum hardness in HAZ at the high diffusible hydrogen content.The cold crack sensitivity in-creases with the increase of the diffusible hydrogen content in the test maximum hardness scope.