The invention provides a beam column connection piece, a building framework structure and a mounting method. The beam column connection piece comprises a beam column connection piece body, a stand column splicing head, and a beam splicing head, which are integrally formed; the beam splicing head comprises a first supporting part for directly supporting a lower flange of a beam, and a first fixing part which is arranged above the first supporting part, is horizontally and convexly arranged on the lateral face of the beam column connection piece body, and is fixedly connected with a web of the beam; a distance between the first supporting part and the first supporting part in a vertical direction is more than the thickness of the lower flange of the beam; the width of the first fixing part in the vertical direction is less than a distance between the two opposite faces of the two flanges of the beam; and the beam column connection piece is further provided with a stopping part of the end part of a stand column. The beam column connection piece has the advantages in that the end part of an H-shaped or transverse H-shaped beam has no need of being machined and has more connection and fixation positions; the connection nodes are stable and reliable; the strength and the rigidity of the connection position are good; and the building framework structure has the advantages of convenience in installation, less process and low labor intensity.