The embodiment of the invention discloses a U rib plate unit assembling and welding method. The method comprises the following steps of S1, fixing a base plate; S2, putting U ribs onto the base plate;S3, operating pressing and positioning mechanisms to the welding start ends of the U ribs to press and position the U ribs; and S4, putting a welding device inside or outside the U ribs to conduct welding at angle joints between the U ribs and the base plate from the welding start ends of the U ribs to the welding terminal ends of the U ribs, wherein in the welding process, the pressing and positioning mechanisms move to the welding terminal ends to complete assembling of the U ribs, moving and welding at the same time while maintaining pressing and positioning on the U ribs. Through the unitassembling and welding method provided by the invention, the assembly accuracy of the U ribs can be ensured; moreover, except positioning welding required during conventional assembling of the U ribs, the welding quality problem caused by positioned welding is eliminated, the welding quality is improved, and the production efficiency is improved.