The invention relates to a longitudinal connection structure and a construction method of the main beam at the expansion joint of a steel-concrete composite bridge. The structure comprises a concretebridge deck slab, a steel beam and a U-shaped steel hinge; the concrete bridge deck slab is composed of a concrete layer, steel bars and an expansion joint; the steel bars are poured into the concretelayer; the expansion joint is arranged at the end of the concrete bridge deck slab; the steel beam supports the concrete bridge deck slab, and is composed of a section steel, a stiffening rib and a shear key; the stiffening rib is welded on the section steel; the shear key is welded on the top of the section steel, and is embedded in the concrete layer; the U-shaped steel hinge is connected withthe ends of the beam on both sides of the longitudinal adjacent steels. The longitudinal connection structure and the construction method of the main beam at the expansion joint of a steel-concrete composite bridge have the advantages of simple structure, firm and convenient construction, and can adopt assembled type of work and be constructed by using a U-shaped steel hinge connection, in which the joint mode of the expansion joint position is changed to a semi-rigid hinge joint, thereby improving the connecting stiffness of the expansion joint of the assembled steel-concrete composite bridgeand alleviating the local warping of the expansion joint and prolonging the service life of the expansion joint.