The invention discloses a general steel formwork assembling device for a concrete firewall. The general steel formwork assembling device comprises two plane elements which are arranged parallelly, have the same structures and are perpendicular to a ground base. Each plane element comprises a plurality of rows of structures. Each row of structures is connected above the adjacent row of structures. Each row of structures is formed by splicing a plurality of main formworks. Vertical elements which are arranged oppositely and have the same structures are arranged between the end pars of the plane elements which are arranged parallelly and have the same structures. Each vertical element comprises a plurality of end formworks which are superimposed one by one. The end formworks are used for being connected with the corresponding main formworks in the two plane elements. The height of the plane elements is the same with the height of the vertical elements. The invention simultaneously discloses an assembling method of the general steel formwork assembling device for the concrete firewall. The universality, recyclability and convenience of the set of steel formwork and a supporting system of the steel formwork bring high economic and social benefits.