The invention relates to the technical field of energy materials, and in particular relates to a preparation method of iron phosphate. Relatively pure iron and a small amount of iron oxide are used as raw materials to react with phosphoric acid to obtain iron phosphate which is then water-washed and oven-dried to obtain an iron phosphate product with uniform particle size and excellent tap density. The preparation method of iron phosphate, provided by the invention, dispenses with the treatment of the byproduct salts generated during iron phosphate production from phosphate and an iron salt; the preparation method controls the properties of iron phosphate through a high pressure oxidation process, so as to effectively improve the tap density and other properties; and the preparation method effectively reduces material loss, reduces the phosphorus content in sewage and reduces the difficulty of sewage treatment through rectification, purification and reuse of phosphoric acid, thus achieving the purpose of green production.