The invention belongs to the technical field of building construction and particularly relates to a polyphenyl granule foam concrete light-weight steel building and a construction method. The polyphenyl granule foam concrete light-weight steel building comprises light-weight steel keels, an outer structural slab, an inner structural slab, polyphenyl granule foam concrete and an outer wall decorative plate, wherein the light-weight steel keels are arranged at intervals and are of a C-shaped structure; the outer structural slab and the inner structural slab are arranged on the two sides of the keels correspondingly, and the outer structural slab and the inner structural slab can be detached; the polyphenyl granule foam concrete is grouted between the outer structural slab and the inner structural slab; and the outer wall decorative plate is arranged on the outer side of the outer structural slab or the outer side of the polyphenyl granule foam concrete. By the adoption of the polyphenyl granule foam concrete light-weight steel building and the construction method, the structure is simple, heat insulation and preservation can be achieved, and sound insulation and fire prevention are achieved.