A beam structure includes a lower cover plate, an upper cover plate and two side web plates jointed between the upper cover plate and the lower cover plate; wherein the two side web plates are arranged perpendicularly to the upper cover plate and the lower cover plate, respectively, to form a hollow box-shaped structure; rib plates protruding outwardly with a plurality of plug heads are welded within the box-shaped structure; the plug heads are respectively plugged in the upper cover plate, the two side web plates and the lower cover plate, and each plugging joint is fixed by laser-MAG hybrid welding from outside of the box-shaped structure. The method includes: assembling two side web plates, rib plates and a lower cover plate on an upper cover plate to form a box-shaped structure; and welding each of joints from outside of the box-shaped structure by laser-MAG hybrid welding.