The invention discloses a method for dismantling a concrete box girder of an under-passing closed frame culvert. The method for dismantling the concrete box girder of the under-passing closed frame culvert comprises the following steps of early-stage preparations, erecting full framing, erecting steel pipe column structures, laying a gantry crane track beam, installing a gantry crane, cutting mainbeam flange plates by adopting a rope saw, eliminating discarded flange plate blocks through hoisting by adopting the combination of the gantry crane and a truck-mounted crane, cutting a main beam box chamber by adopting the rope saw, eliminating discarded main beam box chamber beam blocks through hoisting by adopting the combination of the gantry crane and the truck-mounted crane, and transporting the discarded beam blocks to a designated site by adopting a transportation truck. According to the method for dismantling the concrete box girder of the under-passing closed frame culvert, throughthe design of a temporary support and the cutting steps, the concrete box girder is dismantled by adopting the combination of the truck-mounted crane and the gantry crane; safe and rapid dismantlingof a bridge is achieved; and the method is applicable to rapid dismantling construction of the concrete box girder of the under-passing closed frame culvert in a city bridge.