The invention relates to a lightweight assembled steel-structure house and a building method thereof. The house comprises floors, walls, a door and windows, wherein each floor comprises a first steel-structure frame, a first reinforcing
steel bar strainer, a first concrete layer, a second reinforcing
steel bar strainer, a second concrete layer, a damping and sound-
insulation layer and
butt joint racks, the first reinforcing
steel bar strainer and the first concrete layer are arranged on one side of the first steel-structure frame, the second reinforcing steel bar strainer and the second concrete layer are arranged on the other side of the first steel-structure frame, the damping and sound-
insulation layer is arranged between the first reinforcing steel bar strainer and the second reinforcing steel bar strainer, and the
butt joint racks are arranged at two ends of the first steel-structure frame; each wall comprises a second steel-structure frame, two
layers of parallel reinforcing steel bar strainers arranged in the second steel-structure frame, a thermal-
insulation layer arranged between the two
layers of parallel reinforcing steel bar strainers, a
cement outer wall layer and a fireproof inner wall layer, the
cement outer wall layer and the fireproof inner wall layer are poured on two sides of the thermal-insulation layer, and the walls are connected through connecting columns. The lightweight assembled steel-structure house has the advantages that the walls, the floors and the connecting columns are prefabricated in factories, the walls are connected with the connecting columns through bolts, the walls are connected with the floors through bolts, field construction procedures are simplified, and construction efficiency and house quality are increased.