Abstract: X80 steel was welded by copper liner automatic external root welding.The quality of weld formation and the copper content, microstructure and mechanical properties of the copper bonded and non-copper bonded areas were studied. The results show that the weld was well formed and no defects such as cracks or incomplete fusion were found. The copper content in the non-copper bonded area was consistent with the base metal. The copper mass fraction of the copper bonded area was about 0.46%, and the copper infiltration depth was about 0.18 mm. The microstructure of the weld in the non-copper bonded area was mainly composed of acicular ferrite and a small amount of pre-eutectoid ferrite, while that of the copper bonded area was mainly composed of block ferrite, granular bainite, and free copper. The hardness variation trend of the non-copper bonded area and the copper bonded area was basically the same. The hardness of the copper bonded area of weld root was slightly higher than that of the non-copper bonded area. The tensile strength of the copper bonded area and the non-copper bonded area was 730, 732 MPa, respectively. The impact properties of the two areas were basically the same, and no cracks were found after bending test. The copper infiltration at the root of the weld had no obvious effect on the mechanical properties of the joint.