The invention discloses a combined steel die structure capable of shaping a plate height reducing part and an installation method of the combined steel die structure. The combined steel die structure comprises steel templates mounted at the plate height reducing part, wherein the steel templates are connected in sequence to form a whole body and then form a polyhedral structure; supporting frames are arranged at the outer part of the polyhedral structure; the supporting frames are connected with side walls of the steel templates; a pre-supporting template is arranged at the outer parts of the supporting frames; the supporting frames are arranged between the pre-supporting template and the steel templates; a jacking frame is arranged under the polyhedral structure; the jacking frame is in contact with the steel templates; tensioning holes are formed in the side walls of the steel templates; tensioning devices are arranged in the tensioning holes; the tensioning devices penetrate through the tensioning holes and the supporting frames, and then are fixed with the steel templates and the pre-supporting template. With the adoption of the structure, an edge corner is smooth and straight, a vertical surface is vertical and a cross section is tidy after concrete is poured; a combined steel die can be used infinitely so that the cost is reduced; the problems of an existing die structure used in the plate height reducing parts including balconies of kitchens and bathrooms that the discharging is inaccurate, the deformation is great, the die is easy to damage in a detaching process, and the quality does not reach the standards after pouring are solved.