The invention relates to a building material and in particular relates to a sanding floor for a light-weight steel building and a preparation method thereof. The sanding floor comprises a bottom slurry layer, one layer of non-woven fabric, three layers of glass fiber fabrics, a medium material layer, three layers of glass fiber fabrics and a sanding layer in sequence from bottom to top and is shown in the accompany drawing in the description in detail. The bottom slurry layer is prepared from 100 parts of light burned magnesium oxide, 40 to 60 parts of magnesium sulfate heptahydrate, 50 to 70 parts of water, 20 to 60 parts of talcum powder, 6 to 15 parts of latex powder, 1 to 5 parts of modifying agent and 3 to 10 parts of any one of iron oxide red, iron oxide yellow, iron oxide green, iron oxide blue and carbon black. The medium material layer is composed of 100 parts of the light burned magnesium oxide, 40 to 80 parts of the magnesium sulfate heptahydrate, 60 to 100 parts of the water, 20 to 60 parts of sawdust, 4 to 40 parts of other fillers, 1 to 5 parts of modifying agent and 1 to 10 parts of any one of the iron oxide red, the iron oxide yellow, the iron oxide green, the iron oxide blue and the carbon black. The sanding layer is composed of 100 parts of the light burned magnesium oxide, 40 to 80 parts of the magnesium sulfate heptahydrate, 60 to 100 parts of the water, 20 to 60 parts of wood meal, 0.5 to 2 parts of de-foaming agent, 1 to 5 parts of modifying agent and 1 to 10 parts of any one of the iron oxide red, the iron oxide yellow, the iron oxide green, the iron oxide blue and the carbon black.