Characteristics of the molten pool of the oscillating arc GMAW welding in the allposition vertical welding section were studied, and a set of pipeline allposition welding visual monitoring system was designed. Clear images of the molten pool sequence were collected; the variation characteristics of the vertical welding molten pool of an arc oscillation period were analyzed;the mathematical morphology algorithm was used to obtain the contour of the molten pool. The results show that the defined molten pool visual characteristic parameters can be used to effectively distinguish the weld molten pool of different vertical welding zones during allposition welding process. Under the action of the oscillating arc, the characteristics of verticalupward welding and verticaldownward welding molten pool are obviously different, the length and area of the verticalupward welding molten pool are 21% and 10% larger than those of the verticaldownward welding molten pool, the tail line of the verticalupward welding molten pool is horizontally oriented,while the tail of the verticaldownward welding molten pool exhibits a distinct concave feature.