A preparation method of battery composite material includes steps of providing a manganese-contained compound, phosphoric acid, a lithium-contained compound, a carbon source, and deionized water; processing a reaction of the manganese-contained compound, the phosphoric acid, and a portion of the deionized water to produce a first product; placing the first product at a first temperature for at least a first time period to produce a first precursor, wherein the chemical formula of the first precursor is written by Mn5(HPO4)2(PO4)2(H2O)4; and processing a reaction of at least the first precursor, the lithium-contained compound, and another portion of the deionized water, adding the carbon source, and then calcining to produce battery composite material. Therefore, the preparation time is shortened, the energy consuming is reduced, the phase forming of the precursor is more stable, and the advantages of reducing the cost of preparation and enhancing the quality of products are achieved.