A preparation method of a lithium nickel manganese oxide cathode material of a battery includes steps of providing a nickel compound, a manganese compound, a first quantity of lithium compound, a second quantity of lithium compound and a compound containing metallic ions, mixing the nickel compound, the first quantity of lithium compound, dispersant and deionized water to produce first product solution, adding the manganese compound into the first product solution and mixing to produce second product solution, performing a first grinding to produce first precursor solution, mixing the second quantity of lithium compound, the compound containing the metallic ions and the first precursor solution, then performing a second grinding to produce second precursor solution, and calcining the second precursor solution to produce the lithium nickel manganese oxide cathode material of the battery, the formula of which is written by Li1.0+xNi0.5Mn1.5MyO4. Therefore, the activation energy of reaction can be reduced.