The invention discloses a lithium manganese nickel oxide cathode material having nickel manganese concentration gradient and a preparation method thereof. The average chemical composition of the lithium manganese nickel oxide cathode material can be shown as a molecular formula of LiNi<0.5-x>Mn<1.5+x>O<4>, wherein x is more than or equal to 0.1 and less than or equal to 0.35, the concentration of Ni is gradually reduced in a gradient distribution manner from the particle center to the particle surface of the lithium manganese nickel oxide cathode material, and the concentration of Mn is gradually reduced in a gradient distribution manner from the particle center to the particle surface of the lithium manganese nickel oxide cathode material. The preparation method comprises the following steps of firstly, synthesizing a spherical-like particle having a core-shell structure by a co-precipitation process; and finally, preparing the lithium manganese nickel oxide cathode material with change on the nickel and manganese concentration by element diffusion during the high-temperature roasting process. The cathode material disclosed by the invention has excellent high-temperature cycle stability and rate performance, high reversible capacity, high chemical stability, long cycle life, and excellent comprehensive electrochemical performance.