Disclosed are a middle rail box girder for a large-tonnage bridge crane and a manufacturing method of the middle rail box girder. Two semicircular reinforcing plates are welded to each of the two sides of a large transverse rib plate and a small transverse rib plate of the box girder, the upper end is machined overall, thus a smooth plane is formed, and the part, within a certain range at the bottom of a rail, of the plane is not welded to an upper flange plate and is only in close contact with the upper flange plate. The magnitude of extrusion stress between the upper flange plate and the transverse rib plates can be achieved by changing the thickness of the semicircular reinforcing plates, therefore, the technical bottleneck of use limitation of the middle rail box girder is broken through, the use range of the middle rail box girder is widened, thus the middle rail box girder is suitable for all elevating capacity types, the manufacturing method is simple, meanwhile, the condition that a large bent plate structural style with a half shaft sleeve is adopted for the end of the box girder is suggested for the first time, and economic low-height column hinge connection of the box girder and a trolley is achieved.