Abstract: Influence of porosity and crack defects on impact toughness of X80 steel pipeline steel weld by multi-wire submerged are welding with inner four wires and outer five wires was studied. Due to the limitation of crack preparation technology, 1 / 4 specimen was used as impact toughness specimen. By comparing with full-scale specimen, it was found that the proportional coefficient of impact absorbing energy between 1 / 4 specimen and full-scale specimen was 0.17 at -10^\circ \mathrmC. The results of study on the interaction between crack and porosity showed that porosities with diameters of 0.5 mm and 1 mm reduced the impact toughness without crack arresting effect. For the pre-made cracks perpendicular to the crack propagation path, as long as its position was in the zone 1 mm far from the impact location, the plastic deformation could improve crack propagation resistance, curve the crack propagation path and increase impact absorbing energy. The effect of transverse pre-made crack was equivalent to that of large-diameter air hole. A large amount of plastic deformation could occur on the lower crack surface, which had a certain crack arresting effect and improved impact absorbing energy of samples. The impact absorbing energy of X80 pipeline steel weld was changed by the existence of defects in the weld, while the microscopic fracture analysis results showed that fracture behavior was not changed, and microtopography was still dimple type, which was consistent with the defect-free samples.