The invention discloses an online upward multi-wire submerged-arc welding process for a medium longitudinally-welded steel pipe, and a used welding pad roll, and relates to an online upward multi-wire submerged-arc welding process for a medium longitudinally-welded steel pipe. The invention mainly aims at solving the problems of low product quality and the like because longitudinal pre-precise-welding is adopted when a steel pipe is longitudinally welded in production at present. The online upward multi-wire submerged-arc welding process comprises the following steps: flattening a coiled plate and feeding the flattened steel plate into steel pipe pre-forming equipment at first; bending upwards the flattened coiled plate and keeping a closed opening upward; feeding the flattened coiled plate into steel pipe precise-forming equipment and gradually closing the closed opening; arranging a welding pad roll device in the steel pipe and enabling the welding pad roll on the welding pad roll device to be located just below a completely-closed position of the closed opening of the steel pipe; externally welding at the position of the closed opening in the outer wall of the steel pipe above the welding pad roll by virtue of multi-wire submerged-arc welding; cutting off the steel pipe after welding, taking off the steel pipe and internally welding the steel pipe; arranging the steel pipe on a rotary trolley and keeping a welding seam of the steel pipe downward; welding an inner seam of the steel pipe by virtue of multi-wire submerged-arc welding and through a detection arm; and finally feeding the welded steel pipe into straightening equipment and straightening, obtaining a finished product after end-flattening and a hydrostatic test, and warehousing. The online upward multi-wire submerged-arc welding process has the advantage of high product quality.