Abstract: Cold crack sensitivity and the fracture characteristic of the root pass welding of X80 pipeline steel CO₂ gas shielded arc weld with solid wire was investrgated by implant testing.The results indicate that the X80 steel has the excellent resistance to cold cracking under the preheating 100℃ condition, the critical breaking stress σ_cr(624 MPa) is equivalent to its tensile strength Rm. When the tension stress is higher than Rm, the expiration break would happen, and the breaking nature is the ductile fracture.Under the condition of CO₂ gas shielded arc weld for root pass, the good resistence to crack characteristic of X80 steel has not only been related with the welding procedure with the ultra low hydrogen welding, but also with the lower crack sensitive coefficient(0.17%) as well as the unobvious quench hardening of HAZ.The microstructure of superheat section, with the maximum hardness degree of 297 HV is mainly the block ferrite.