Fabricated building structure system and construction method

The invention relates to the technical field of buildings, in particular to a fabricated building structure system and a construction method. The fabricated building structure system comprises prefabricated columns, a prefabricated beam, a prestressed structure and connecting reinforcements. Column cavities are formed in the prefabricated columns, a beam cavity is arranged in the prefabricated beam, and the prestressed structure is arranged in the beam cavity. The ends of the prefabricated beam are in lap joint with the ends of the prefabricated columns. One ends of the connecting reinforcements are connected with the prefabricated columns, and the other ends of the connecting reinforcements are at least partially located in the beam cavity. The column cavities and the beam cavity are filled with cast-in-place concrete layers. Concrete is cast in the column cavities and the beam cavity in site, and therefore forming columns and a forming beam are formed. The forming columns, the forming beam and connecting jo