The invention discloses a preparation method of iron phosphate. The method is as below: using an iron powder or iron oxide as an iron source, and phosphoric acid as a phosphorus source for the preparation of an iron phosphate mother liquor, then adding hydrogen peroxide for the preparation of an iron phosphate crude product, and finally washing the iron phosphate crude product by using distilled water, drying the product after washing to obtain the iron phosphate product, and recycling the phosphoric acid filtrate. The preparation method of iron phosphate directly uses iron powder or iron oxide as the raw material, is easier to control the content of trace metal and reaches more stable tap density index in comparison with the production of iron phosphate from the phosphate and iron salt; the phosphoric acid used as phosphorus source can be recycled; the iron phosphate filtrate as phosphorus source is recycled to the preparation of ferrous phosphate, so as to effectively reduce the content of phosphorus in wastewater, and improve the utilization rate of phosphorus source.