A preparation method of a battery composite material includes steps of providing phosphoric acid, manganese carbonate, water and a first reactant; processing a reaction of the phosphoric acid, the manganese carbonate and the water to produce a first product; calcining the first product to produce a precursor, which is written by Mn2P2O7; processing a reaction of the precursor and at least the first reactant to get a reaction mixture, and then calcining the reaction mixture to produce the battery composite material. As a result, the present invention achieves the advantages of reducing the times of the reduction-oxidation reaction, so that the stability of the processes is enhanced, and the difficulty of the processes is reduced.