The invention relates to a 45 degrees beam and column combined buckling piece and a light steel structure. The combined buckling piece comprises a chord member connecting section, a left diagonal web member connecting section and a right diagonal web member connecting section; stamped ribs are stamped on the combined buckling piece; the left diagonal web member connecting section and the right diagonal web member connecting section are distributed at an angle of 45 degrees; a plate body between the left diagonal web member connecting section and the right diagonal web member connecting section is bent downward to form a circular arc limiting turned-up edge; plate bodies at the external side surfaces of the left diagonal web member connecting section and the right diagonal web member are both bent downward to form nipped-waist arc limiting turned-up edges; by using the circular arc limiting turned-up edge and the nipped-waist arc limiting turned-up edges, the diagonal web members are limited within a clamping groove formed by the circular arc limiting turned-up edge and the nipped-waist arc limiting turned-up edges; and the stamped ribs are arranged to be parallel to the edges of the plate bodies and communicated with each other to form a closed whole body. According to the light steel structure formed by the combined buckling piece, the diagonal web members are continuously arranged between an upper chord member and a lower chord member and buckled with each other by 45 degrees beam and column combined buckling pieces at the nodes. The 45 degrees beam and column combined buckling piece and the light steel structure provided by the invention have the advantages of reasonable stress, good integrality, intensive production and construction and the like and can be widely applied to the combination of the light steel beams or columns.