Disclosed is a large span prefabricated monolithic casting structure. The large span prefabricated monolithic casting structure comprises a floorslab, a superposed beam and a column, the superposed beam and the column are used for supporting the floorslab, the superposed beam is divided into a short frame main beam, a long frame beam and an auxiliary beam, the auxiliary beam is connected with the main beam, the superposed beam comprises a prestress prefabricated beam and a cast-in-situ portion, the prestress prefabricated beam comprises a beam body and a pre-stressed steel strand which is buried in the beam body and serves as a main rib, two ends of the short frame main beam and the long frame beam are connected with the column, cross sections of prefabricated beam portions of the long frame beam and the auxiliary beam are invertedly T-shaped, the superposed beam after monolithic casting is an I-shaped beam connected with the floorslab, and the auxiliary beam and the long frame beam utilize the inverted T-shaped prestress prefabricated beams with small intervals. The large span prefabricated monolithic casting structure has the advantages that the span, the thickness and the self weight of the floorslab are small, heights of the beams are reduced, reinforcing bars are reduced, and the short main beam which uses a superposed rectangular beam is good in shear resistance and convenient to connect with the auxiliary beam.