The invention discloses a method for preparing magnesium hydrate crystal whiskers and calcium sulfate with phosphate tailings as the raw material. Phosphate tailings obtained through flotation are mixed with hydrochloric acid for reaction, and purification and filtration are conducted to obtain a mixed solution containing magnesium chloride and calcium chloride; phosphate tailings and sulfuric acid are mixed for reaction, and purification and filtration are conducted to obtain a magnesium sulfate solution; the two solutions are mixed for reaction, and filtration is conducted to obtain calcium sulfate sediment and magnesium chloride filtrate; alkali liquor is added to the magnesium chloride filtrate for reaction and seasoning, and then a liquid-solid mixture containing basic magnesium chloride crystal whiskers is obtained; then an alkaline solution is added for reaction and seasoning, so that the magnesium hydrate crystal whiskers are obtained. By the adoption of the method, the nanometer or micron magnesium hydrate crystal whiskers are prepared with phosphate tailings as the raw material, the high-quality byproduct calcium sulfate is also obtained, and high-value application of solid waste in the phosphorus chemical industry is achieved.