The invention discloses a composite beam, which comprises reinforcement concrete prefab beam and section steel joints, wherein both ends of reinforcements of the reinforcement concrete prefab beam are connected with section steels, one end of section steel joint is pre-imbedded into the wall pier concrete and is connected to the reinforcement inside the wall pier, the other end of the section steel joint is remained outside the wall pier, both ends of section steel of the prefab beam and the section steel joints are connected to form the composite prefab beam. Due to the composite beam, the section steel joints and the wall piers are integrated with good connection integration property of the beams and simple process, wherein both ends of the section steel is connected with the section steel joints of the prefab beam to form composite beam, the connection between the prefab beam end and the section steel joints adopts welding, bolt connection type, and the like, the composite beam production can be industrialized, needs no framework, has small materials consumption and rapid construction progress.