The present invention relates to a mold-prefabricated beam structure comprising: a plurality of main angles comprising longitudinal members having inverted L-shaped cross sections, and arranged in the direction parallel to the longitudinal direction of a beam at a position corresponding to each corner of the beam; hoop angles arranged in the direction intersecting with the longitudinal direction of the beam at each side of the beam, and of which the end portions overlapping at the corner of the beam are fixed to each other so as to encompass the plurality of main angles; and mold panels respectively arranged at both side surfaces and a bottom surface of the beam and fixed to the hoop angles.