The invention discloses a method for preparing calcium sulfate whisker by-product magnesium hydroxide and magnesium ammonium nitrate by decomposing phosphate tailings by nitric acid. The method comprises the following steps: 1) adding water into phosphate tailings to prepare phosphate tailing pulp, adding a nitric acid solution, carrying out an acidolysis reaction, and filtering reaction slurry toobtain a mother liquor I and a filter residue I; 2) adding sulfuric acid or a sulfate into the mother solution I, carrying out a reaction, then carrying out cooling crystallizing, and carrying out filtering separation to obtain a filter cake II and a mother liquor II, washing and drying the filter cake II to obtain calcium sulfate whiskers, and bringing the mother liquor II back to the step 1) for an acidolysis reaction; 3) adding a calcium salt into a mother solution II subjected to multiple times of reaction, carrying out a reaction, and carrying out filtering separation to obtain a filterresidue III and a mother liquor III; and 4) introducing ammonia into the mother liquor III, carrying out a reaction, carrying out filtering separation to obtain a filter cake IV and a mother liquor IV, washing and drying the filter cake IV to obtain a magnesium hydroxide finished product, adding nitric acid into the mother liquor IV, carrying out a reaction, and carrying out concentration and granulation on the mother liquor IV to obtain the byproduct magnesium ammonium nitrate. According to the invention, comprehensive utilization of phosphorus tailing resources is realized, waste is changedinto wealth, and the green chemical concept is conformed.