The invention belongs to the field of chemical engineering and particularly relates to a method of decomposing phosphate tailings by hydrochloric acid to produce Mg(OH)2 and CaSO4. The method includes the steps of: soaking the phosphate tailings in hydrochloric acid and water to perform a complete mixing reaction, and performing primary filtration to obtain a mother liquid (I); adding diluted sulfuric acid to the mother liquid (I) and performing secondary filtration to obtain a mother liquid (II) and filter cake, water-washing and crystallizing the filter cake to generate CaSO4 crystal whiskers; adding ammonia water to the mother liquid (II) to perform primary and secondary neutralization to obtain a Mg(OH)2 product and a mother liquid (IV); and mixing the mother liquid (IV) with residue (II), which is generated in primary neutralization, to produce ammonium chloride-phosphate. With the phosphate tailings and hydrochloric acid as raw materials, the method can produce the Mg(OH)2 and CaSO4 and can by-produce the ammonium chloride-phosphate. The whole process is simple and is in a total-closed circulating manner. The method can turn waste into resources and achieves clean production.