Abstract: The cold crack sensitivity analysis of the copper backing outside root welding joints of X80M pipeline steel with large thickness and diameter under different assembly conditions was carried out. The effect of different assembly conditions on the cold cracking tendency of the joint was studied using the carbon equivalent method and stress field simulation. According to the results, the heat affected zone (HAZ) at the top cover layer and the root weld layer zone at the bottom of the X80M pipe steel weld joints are the weakest areas, which have the greatest cold crack tendency. When the butt gap is increased from 0 − 0.5 mm to 0.6 − 1 mm, the relative carbon equivalent of the root weld layer in the weld zone is also increased by 16.6%, which increases the cold cracking tendency. The increase in misalignment increases the hardenability of the heat affected zone of the joint cover layer and increases the susceptibility to cold cracking. The increase in the misalignment and the butt gap will lead to a further increase in the residual tensile stress after welding, and the increase in the misalignment will also lead to stress concentrations in the misalignment area of the joint, and the increase in the misalignment from 0 mm to 3 mm will result in a 52.4% increase in the peak residual stress at the concave corner of the root weld layer in the weld zone. Therefore, the X80M pipeline steel arc welding process needs to minimize the butt gap and the amount of misalignment to reduce the tendency of cold cracking.