The invention discloses a preparation method for a lithium ion battery cathode material-lithium nickel manganese oxide. The method adopts oxalate and hydroxide as precipitants at the same time for preparing lithium nickel manganese oxide through a coprecipitation method and a solid-phase sintering method. The method comprises the following steps: synthesizing a lithium nickel manganese oxide precursor by double precipitants, that is, synthesizing an oxalate/nickel hydroxide compound precursor material by controlling the pH value of a reaction solution according to the characteristic that concentration product values of oxalate and nickel hydroxide are similar; mixing the precursor material with a lithium source; sintering to synthesize the cathode material-lithium nickel manganese oxide. According to the method, the step of adopting only hydroxide as the precipitant under inert gas shielding is removed, so that the production cost is reduced; compared with lithium nickel manganese oxide prepared by taking only oxalate as the precipitant, the tap density is effectively improved; as the concentration product values of oxalate and nickel hydroxide are similar, the more uniform oxalate/nickel hydroxide compound precursor material can be obtained according to the control on the pH value of the solution.