The invention relates to the field of buildings, in particular to a light steel building. The light steel building is formed by the way that building blocks and light steel keels are assembly into a bounding wall, and is composed of pillars composed of light steel keels, a floor frame, a roofing frame and a ring beam. The invention is assembled by external wall body building blocks (2), U-shaped light steel keels (3), L-shaped light steel keels (4), series bolts (5), inner wall body building blocks (6), light steel bars (8), heat preservation material (9), pillars (10), an L-shaped light steelkeel connecting piece (11), a fastening piece (12), a truss (13), a window purline (41) and a door purline (42). The invention is characterized in that the vertical surfaces of the external wall bodybuilding blocks (2) and the inner wall body building blocks (6) are parallel to form an interlayer space (7), and the pillars (10), the floor frame, the roofing frame and the ring beam are composed of light steel angle channel (701).The invention has firm and reliable structure, high energy saving efficiency and safe and quick construction, can realize industrialized industry scale and better cost performance.