The invention discloses a method for preparing high-purity magnesium fluoride by using magnesite. The method is characterized by comprising the following steps: (1) calcining the magnesite, thus obtaining light calcined magnesia mineral powder; (2) taking and adding the light calclined magnesia mineral powder in an ammonium salt solution to remove soluble calcium, and filtering, thus obtaining filter cake, wherein ammonium salt is one or several of ammonium chloride, ammonium nitrate, ammonium sulfate, ammonium oxalate or ammonium acetate; (3) adding the obtained filter cake in an organic acid solution for reacting, and filtering after reacting, thus obtaining leachate, wherein an organic acid is one or several of a formic acid, an acetic acid, a propanoic acid, a lactic acid, a citric acid, a glycolic acid, a malonic acid, a succinic acid or an adipic acid; (4) adding a hydrofluoric acid in the leachate, generating a precipitate, filtering, washing, and drying, thus obtaining the high-purity magnesium fluoride. According to the method disclosed by the invention, the source of raw materials is wide, the cost is low, the requirements of a device are low, the high-purity magnesium fluoride can be obtained by utilizing cheap magnesite, and the production cost of the high-purity magnesium fluoride is reduced to a large extent.