A modular integrated building, comprising a plurality of prefabricated room units assembled together in the same floor and in adjacent floors, each prefabricated room unit being prefabricated integrally with a top plate, a bottom plate and a wall body, with all or part of the prefabricated room units being further prefabricated integrally with a load-bearing structural wall or column, wherein a top of each prefabricated room unit is provided with reserved, exposed reinforcing bars, which include first vertical reinforcing bars and horizontal reinforcing bars, and a bottom of the load-bearing structural wall or column of the prefabricated room unit is provided with a semi-prefabricated connecting port; wherein the semi-prefabricated connecting port extends to a bottom surface and a side wall of the prefabricated room unit, and is provided with reserved, exposed reinforcing bars therein, which include second vertical reinforcing bars and lateral stirrups, and so that the first vertical reinforcing bars arranged at the top of the prefabricated room unit of a next floor are inserted in the semi-prefabricated connecting port and thus connected with the second vertical reinforcing bars and lateral stirrups arranged therein, the prefabricated room units of two adjacent floors being connected with each other through in-situ casting concrete in the semi-prefabricated connecting port; and wherein a cast-in-situ concrete interlayer is arranged between the bottom plate of the prefabricated room unit of a floor and the top plate of the prefabricated room unit of a next floor, for connecting adjacent prefabricated room units of a same floor together, and reinforcing bars of the cast-in-situ concrete interlayer are connected with the horizontal reinforcing bars arranged at the top of the prefabricated room unit of the next floor. wherein a top of each prefabricated room unit is provided with reserved, exposed reinforcing bars, which include first vertical reinforcing bars and horizontal reinforcing bars, and a bottom of the load-bearing structural wall or column of the prefabricated room unit is provided with a semi-prefabricated connecting port; wherein the semi-prefabricated connecting port extends to a bottom surface and a side wall of the prefabricated room unit, and is provided with reserved, exposed reinforcing bars therein, which include second vertical reinforcing bars and lateral stirrups, and so that the first vertical reinforcing bars arranged at the top of the prefabricated room unit of a next floor are inserted in the semi-prefabricated connecting port and thus connected with the second vertical reinforcing bars and lateral stirrups arranged therein, the prefabricated room units of two adjacent floors being connected with each other through in-situ casting concrete in the semi-prefabricated connecting port; and wherein a cast-in-situ concrete interlayer is arranged between the bottom plate of the prefabricated room unit of a floor and the top plate of the prefabricated room unit of a next floor, for connecting adjacent prefabricated room units of a same floor together, and reinforcing bars of the cast-in-situ concrete interlayer are connected with the horizontal reinforcing bars arranged at the top of the prefabricated room unit of the next floor.