A modular steel structure house comprises a plurality of module units which are spliced with one another. Each module unit comprises a steel structure framework, an outer wall, an inner wall, a floor slab, an electrical system and a water supply and drainage system, wherein the steel structure framework comprises a bottom framework, a top framework and a plurality of portal frames arranged in parallel. Each portal frame comprises portal frame stand columns and a portal frame cross beam, wherein the two ends of the portal frame cross beam are welded to the upper ends of the two portal frame stand columns; the lower ends of the portal frame stand columns are welded to bottom framework side beams; and an upper mounting base is arranged on the top of each portal frame stand column, a butt joint area is formed in the middle of each upper mounting base, and mounting lugs are formed on the two sides of each upper mounting base. A top framework side beam is arranged between the upper ends of every two adjacent portal frame stand columns on the same side. The two ends of each top framework side beam are connected with the corresponding upper mounting bases through bolts. The top framework side beams and the portal frame cross beams form the top frameworks. Each outer wall comprises an outer wallboard and a suspension part, and the outer wall is fixed to the corresponding steel structure framework through the suspension part. According to the modular steel structure house, lightweight design is achieved, the structural strength is high, house splicing can be achieved, and construction is convenient.