Provided is a low-floor bogie outside beam assembling and welding tool and an assembling and welding method thereof. According to the method, it can be achieved that multiple steps of assembling can share one set of tool, box boundary beam assembling, boundary beam inner rib assembling and boundary beam outer body fastening assembling can be achieved at the same time, three-part assembling formed by low-floor boundary beams can be achieved through end ejecting device, a locating pin, a bottom support, a reinforcing beam, a rib plate locating block, an oblique plane rib plate locating block, a cross beam steel pipe locating device and the like, assembling and welding of boundary beams are achieved, and spot welding deformation control and rigid clamping can be achieved. The tool comprises a platform supporting base, a first assembling part used for clamping and locating boundary beam arch structures and a second assembling part used for clamping end bases, locating is achieved through a locating block and bolt ejecting device structure, an end compression locating device, a cover plate pressing device, a box pressing device and a cross beam steel pipe locating device are adopted for clamping, box bodies assembled by the first assembling part is lifted into a groove from a cross beam steel pipe to a box beam to complete locating and pressing of a box, and an upper cover plate is then lifted in to complete assembling and welding.