The invention relates to a preparation method of a LiNiMn<2-x>O<4> cathode material, and belongs to the technical field of lithium-ion battery cathode materials. The method comprises the following steps: firstly, carrying out ball-mill mixing on a lithium source, a nickel source, a manganese source and an assistant for 0.5-10 hours, and drying the mixture at 60-160 DEG C to prepare a precursor, wherein at least one of the lithium source, the nickel source, the manganese source and the assistant is chloride; and secondly, heating the obtained precursor in air or oxygen atmosphere to 500-800 DEG C at the rate of 1-20 DEG C/min, heating the precursor at constant temperature for 5-48 hours, and cooling the precursor to room temperature to obtain the product LiNiMn<2-x>O<4> cathode material, or washing and drying the precursor to obtain the LiNiMn<2-x>O<4> cathode material. According to the method, a raw material chlorination process is provided; and through introducing chloride ions, the LiNiMn<2-x>O<4> (x is less than or equal to 0.5 and greater than or equal to 0.4) material in the solid-solid reaction process is promoted to grow up and expose a {111} crystal plane at low temperature, so that preparation of the high-performance LiNiMn<2-x>O<4> (x is less than or equal to 0.5 and greater than or equal to 0.4) material can be simply and effectively achieved.