The invention provides a big self-supporting combined steel formwork and relates to the technical field of building formworks. The big self-supporting combined steel formwork comprises a portal supporting frame, a right-hand thread adjustment lead screw, a left-hand thread adjustment lead screw and formwork connectors. The portal supporting frame comprises portal frame structures composed of first supports and second supports, and the portal frame structures are connected through reinforcing supports. An operation platform is arranged on the second support at the topmost end of each section of the portal supporting frame. The big self-supporting combined steel formwork further comprises double-hook connectors and a firewall coping mold, wherein adjacent building formworks are connected through the double-hook connectors, and the firewall coping mold is mounted at the top of the building formwork. The invention further provides a construction method for constructing a bare concrete firewall through the big self-supporting combined steel formwork. By the adoption of the construction method, the problems of inclining, slab staggering, mortar leakage and the like caused after building formworks are assembled in the prior art can be solved; the construction speed is high, and mounting is convenient; and operation for field assembly of the steel formwork is easy, the degree of mechanization is high, and the construction cycle is short.