A preparation method of a battery composite material includes steps of providing phosphoric acid, iron powder, a carbon source and a first reactant, processing a reaction of the phosphoric acid and the iron powder to produce a first product, calcining the first product to produce a precursor, among which the formula of the precursor is written by Fe 7 (PO 4 ) 6 , and processing a reaction of the precursor, the carbon source and the first reactant to get a reaction mixture and calcining the reaction mixture to produce the battery composite material. As a result, the present invention achieves the advantages of reducing grind time of fabricating processes, so that the prime cost, the time cost, and the difficulty of fabricating are reduced.