The invention discloses a method for manufacturing a segmental box girder, which mainly comprises the following steps of: (A) prefabricating a first segmental block on a short-line pedestal; (B) prefabricating a second girder block on a long-line pedestal by taking the first segmental block as the matched girder; (C) prefabricating a third block on the long-line pedestal by taking the second girder block as the matched girder and sequentially prefabricating the blocks one by one until circulating to prefabricate the segmental block adjacent to the last segmental block of the box girder, namely, the last-but-one segmental girder block; and (D) prefabricating the last segmental block on the short-line pedestal by taking the last-but-one segmental girder block as the matched girder. The method for manufacturing the segmental box girder adopts a long-line and short-line matched prefabrication process principle so as to greatly shorten the construction period, increase the quality standardof the project, and reduce the error generated in the matched prefabrication process of the box girder.