A preparation method of a battery composite material at least includes the following steps. Firstly, an iron compound, phosphoric acid, a manganese compound, a lithium compound and a carbon source are provided. Then, the phosphoric acid is added to a mixture of the iron compound and deionized water while stirring to form a first phosphate solution, a first amount of the manganese compound is added to the first phosphate solution, and the manganese compound and the first phosphate solution are continuously reacted for a first time period, so that a first product solution is formed. Then, a reaction between the first product solution, the carbon source and the lithium compound is carried out to form a precursor. Then, the precursor is thermally treated to form the battery composite material, wherein the battery composite material has a chemical formula: LiFexMn1-xPO4. Since the product powder is not subjected to aggregation during the thermal treatment process, the electric performance of the battery is enhanced.