At present, all industries have more requirements to conveying pipe products in machinery industry with the development of science and technology. For example, a large amount of big-diameter and high-performance conveying pipes, especially conveying pipes with a diameter of bigger than Φ630 mm are required in oil exploration. However, the steel pipe manufacturers both at home and aboard can only produce conveying pipes with a diameter of less than Φ630 mm, and a single method to produce the conveying pipes with a diameter of less than Φ630 mm has always been used. For example, some foreign manufacturers use FFX forming method, namely the flexible forming method, and some other steel pipe manufacturers both at home and aboard normally use a cage roll type forming machine set. Therefore, it has become a difficult problem for the steel pipe manufacturers both at home and aboard to produce the conveying pipes with a diameter of bigger than Φ630 mm using each of the aforementioned single production methods. In a case where the cage roll forming machine is used to produce the large diameter longitudinal welded pipe, forming rolls are to be distributed on a forming surface of a work piece. Even though the forming effect is good, one of outer rolls has to be heavier than 10 tons which makes it difficult to produce and install such big equipment and leads to high investment, low production efficiency and instable welding quality. In particular, the steel pipe manufacturers both at home and aboard cannot use each of the aforementioned single methods to produce a welded pipe with a diameter of bigger than Φ630 mm, therefore, it is still far from building a production line of welded pipe with a diameter of bigger than Φ630 mm. In order to solve the above problem, it is very necessary to develop a production line of large diameter longitudinal welded pipe by which a welded steel pipe with a diameter of bigger than Φ630 mm can be produced.