The invention relates to a method for preparing calcium sulfate whiskers and magnesium hydroxide by decomposing phosphate tailings through hydrochloric acid. The method comprises the following steps: adding phosphate tailings and hydrochloric acid into a decomposing tank to adequately react in the decomposing tank, and carrying out solid-liquid separation I on reacted slurry, so as to separate first mother solution and solid waste residues, wherein the solid waste residues are acid-insoluble substances; adding sulfuric acid into the first mother solution to react for crystallization, so as to generate the calcium sulfate whiskers; carrying out solid-liquid separation on crystallized slurry, so as to separate calcium sulfate dihydrate whiskers, wherein the mother solution is a mixed mother solution containing magnesium chloride and hydrochloric acid; after magnesium chloride in the mixed mother solution reaches a certain concentration, starting to take out a part of mixed mother solution, carrying out flash evaporation to evaporate HCl in the mother solution so as to obtain the mother solution mainly containing magnesium chloride, and returning the rest part of the mixed mother solution to the decomposing tank; introducing ammonia gas into the magnesium chloride mother solution, and carrying out solid-liquid separation, so as to obtain a relatively pure magnesium chloride mother solution and magnesium ammonium chloride solids containing a small amount of impurities; and absorbing HCl gas in the front stage by a magnesium ammonium chloride solution to enable the pH value of the magnesium ammonium chloride solution to be close to neutrality, and carrying out concentration and crystallization, so as to obtain magnesium ammonium chloride crystals. The method has the advantage that magnesium hydroxide and the calcium sulfate whiskers which have relatively high quality can be produced from industrial waste and the cheap raw materials.