A preparation method of a battery composite material, comprising steps of: (a) providing phosphoric acid, iron powder, a carbon source and a first reactant; allowing deionized water to dissolve a first quantity of said phosphoric acid for forming a first phosphoric acid solution at a first temperature; processing a reaction of said first phosphoric acid solution and said iron powder at a second temperature, lowering the reaction temperature to a third temperature after said second temperature is reached by the reaction temperature, and maintaining the reaction temperature for a first time period; and lowering the reaction temperature to a fourth temperature and adding a second phosphoric acid solution consisting of a second quantity of phosphoric acid, and processing a reaction of said first phosphoric acid solution, said second phosphoric acid solution and said iron powder for a second time period in order to produce a first product, (c) adding said carbon source, grinding and calcining said first product to produce a precursor, wherein the formula of said precursor is written by Fe 7 (PO 4) 6; and (d) processing a reaction of said precursor and said first reactant to obtain a reaction mixture, and then calcining said reaction mixture to produce said battery composite material, wherein said first reactant is a lithium hydroxide, a lithium carbonate, a compound consisting of lithium or a mixture of several compounds consisting of lithium, said first product is an amorphous body of ferric phosphate, and the formula of said first product is written by a-FePO 4 ·xH 2 O, wherein a > 0 and x > 0. (a) providing phosphoric acid, iron powder, a carbon source and a first reactant; allowing deionized water to dissolve a first quantity of said phosphoric acid for forming a first phosphoric acid solution at a first temperature; processing a reaction of said first phosphoric acid solution and said iron powder at a second temperature, lowering the reaction temperature to a third temperature after said second temperature is reached by the reaction temperature, and maintaining the reaction temperature for a first time period; and lowering the reaction temperature to a fourth temperature and adding a second phosphoric acid solution consisting of a second quantity of phosphoric acid, and processing a reaction of said first phosphoric acid solution, said second phosphoric acid solution and said iron powder for a second time period in order to produce a first product, allowing deionized water to dissolve a first quantity of said phosphoric acid for forming a first phosphoric acid solution at a first temperature; processing a reaction of said first phosphoric acid solution and said iron powder at a second temperature, lowering the reaction temperature to a third temperature after said second temperature is reached by the reaction temperature, and maintaining the reaction temperature for a first time period; and lowering the reaction temperature to a fourth temperature and adding a second phosphoric acid solution consisting of a second quantity of phosphoric acid, and processing a reaction of said first phosphoric acid solution, said second phosphoric acid solution and said iron powder for a second time period in order to produce a first product, (c) adding said carbon source, grinding and calcining said first product to produce a precursor, wherein the formula of said precursor is written by Fe 7 (PO 4) 6; and (d) processing a reaction of said precursor and said first reactant to obtain a reaction mixture, and then calcining said reaction mixture to produce said battery composite material,