The invention discloses a magnesite based light slurry for light steel buildings and a preparation method thereof. The slurry contains the following materials and components in parts by weight: 350-500 parts of magnesium oxide, 150-300 parts of magnesium chloride, 1-100 parts of coal ash, 1-50 parts of silica fume, 0.5-1.0 part of hydroxypropyl methyl cellulose, 1-100 parts of filler, 150-350 parts of water, and 7-15 parts of a modifier. According to the invention, the silica fume, the coal ash, the modifier and the like are added to effectively reduce the product halogenide phenomenon, and improve the durability; in order to realize wider application of magnesium oxide and magnesium chloride resources, massive industrial and agricultural waste are turned into wealth, so as to further savethe production cost and protect the environment. According to the invention, magnesite based slurry with certain corrosion resistance and light steel buildings with superstrong corrosion resistance are combined for use, and therefore the problems that since the halogenide phenomenon is easy to occur to the magnesite based slurry of a conventional formula, and certain corrosivity is caused to general steel, the magnesite based light slurry is constrained greatly in the application process.