The invention relates to a preparation method of lithium nickel manganese oxide serving as a high-voltage positive electrode material. The preparation method comprises the following steps of using oxalate and carbonate as precipitants; preparing an oxalic acid/nickel manganese carbonate composite precursor by a co-precipitation method; pre-burning, dispersing into a lithium hydroxide water solution, spraying and drying; calcining at high temperature, so as to obtain the lithium nickel manganese oxide serving as the high-voltage positive electrode material. Compared with the traditional method using the hydroxide as the precipitant, the protection by inert gas is not needed, the strong alkaline is not needed, the requirement on equipment is low, and the production cost is reduced; compared with the traditional method using single oxalate or carbonate as the precipitant, the oxalate and carbonate are used as double precipitants, so that the compaction density of the material is effectively improved. The prepared high-voltage positive electrode material has the advantages that the material is in a sphere shape, the voltage is high, the compaction degree is high, the capacity is high, the doubling property is excellent, the material cost is low, and the like.