Abstract: Based on the Charpy impact toughness data of the girth welded joints of X80 steel pipelines with automatic welding in a domestic project, a systematic statistical analysis was carried out. Meanwhile, the microstructure morphology and crack propagation path of the impact specimens in on-site welded joint inspections were analyzed to clarify the degree of dispersion of the impact data of the girth weld joints of X80 steel pipelines with automatic welding and explore the mechanism of toughness dispersion. The results show that for the fully automatic welding girth welded joints of X80 steel pipelines, the standard deviation of the toughness data in the weld zone is 51.14, and that in the fusion line is 87.99. For the combined automatic welded joints, the standard deviation of the toughness data in the weld zone is 49.59, and that in the fusion line is 62.48. Under both welding processes, the dispersion of the toughness data in the fusion line is bigger than that in the weld zone. The analysis of the microstructure and crack propagation path indicates that the toughness dispersion in the weld zone is closely related to the volume proportions of AF（acicular ferrite） and PF（polygonal ferrite） in the inter-pass microstructure.When the proportion of AF increases, the resistance to crack propagation increases, and the impact value also increases accordingly. The significant microstructure inhomogeneity on both sides of the fusion line causes cracks to easily propagate transgranularly along the coarse GB（grain boundary） structure in the coarse-grained heat-affected zone, resulting in low toughness values. Moreover, the machining accuracy of the notch also affects the crack propagation path, making the crack propagation path deviate from the fusion line, tending towards the base metal or weld microstructure and generating "false" toughness values. These two factors ultimately leads to the appearance of the toughness dispersion phenomenon at the fusion line.