The invention provides a preparation method of ferric phosphate. The preparation method comprises the steps of mixing and filtering a ferric source and an acid solution, adjusting a pH value to 0.5-1 after the obtained filtering liquid and phosphoric acid are mixed, adjusting the pH value to 1.5-2.5 after the mixture and an inducer are mixed, and performing precipitation reaction to obtain the ferric phosphate, wherein the mole ratio of substances of iron ions and the phosphoric acid is 1:1, and the ferric phosphate with good crystallinity still can be enabled to be obtained without an excessive amount of the phosphoric acid. By a two-step alkali regulation method, the control of a reaction environment can be achieved, partial alkalization also can be prevented from forming ferric hydroxide, an excessive amount of phosphate radical is not needed, the increase of phosphorus content in waste water is prevented, and the cost is reduced.