Abstract: Aiming to online repair needs of oil and gas pipeline, optimization of CMT additive repair process was carried out by self-designed low-carbon micro alloy welding wire. Additive manufacturing metal was prepared on X80 steel substrate after service, whose microstructure, microhardness, impact absorbed energy and tensile strength were tested. The results showed that lath bainite and a small amount of quasi-polygonal ferrite appeared in additive manufacturing metal. From additive manufacturing metal to substrate, lath ferrite transformed to granular bainite. There appeared no softening in heat affected zone of X80 steel substrate after CMT additive repair. Microhardness of additive manufacturing metal was 119% that of substrate. Average impact absorbed energy of additive manufacturing metal in different directions at room temperature was about 74% that of substrate. Average elongation of additive manufacturing metal in different directions was about 66% that of substrate. Tensile strengths of additive manufacturing metal from large to small were in the order of scanning direction, lap direction and vertical direction in turn, and tensile strength ranged from 103% to 114% that of substrate. It was suggested that controlling boundary distribution characteristics of passes (fish-scale lines) on the cross section of additive manufacturing metal could reduce its number of high temperature thermal cycles, thus to improve toughness of additive manufacturing metal to a certain extent.Highlights: (1) Microstructure and mechanical properties of cold metal transition (CMT) additive manufactured X80 pipeline steel were systematically studied.(2) It was proposed to improve toughness of additive repair additive manufacturing metal by controlling distribution characteristics of passes on the cross section of additive manufacturing metal.