The invention discloses a prefabricated steel structure external wall with a flat and safe structure, which comprises ALC wall slabs at outdoor; the joint part of the ALC wall slabs is provided with a holding plate, and ALC external hanging wall slab nodes are arranged at one side of each of the bottom part of a first ALC slab at the upper part and the top part of a second ALC slab at the lower part in two connected ALC wall slabs; a structure beam is arranged between two ALC external hanging wall slab nodes; a lining wall is arranged on the structure beam, and the lining wall is fixedly connected with the ALC wall slab. The prefabricated steel structure external wall with the flat and safe structure adopts the narrow structure beam for reducing its thickness and adding the height; thus the enough anti-lateral stability after welding with the structure pillar is guaranteed; through arranging the lining wall thereon, the flat integral structure is effectively guaranteed; the steel structure does not contact with outdoor air medium, and no structure beam is raised at indoor; its beauty is improved; the easy folding problem caused by too high wall slab is well solved, and the construction safety is greatly improved.