The invention discloses a cobalt-free positive electrode material, a preparation method thereof and a lithium ion battery. The molecular formula of the cobalt-free positive electrode material is Li1 + nNixMnyMzO2, wherein x + y + z = 1, 0.5 < = x < 1.0, 0 < y < 0.5, 0 < = z < = 0.1,-0.1 < = n < = 0.5; m is one or more of Al, Mg, Sr, Ti, Fe, Sc, V, Y, Zr, Nb and Mo except Co. The preparation method comprises the following steps: mixing raw materials including a nickel source, a manganese source, a lithium source, an M source, a nucleating agent graphene oxide hydrogel, a precipitator and a complexing agent to prepare a mixed solution, and applying a microwave field in the hydrothermal process of the raw material mixed solution; performing hydrothermal reaction in a protective atmosphere to obtain a positive electrode material process sample; and sintering the process sample in an oxygen-containing atmosphere to obtain the cobalt-free positive electrode material. According to the preparation method, the material performance is improved by regulating and controlling the microstructure and optimizing the preparation process, meanwhile, the cost is saved, and the manufacturing process is simplified; the lithium battery prepared from the cobalt-free positive electrode material has excellent chemical stability and electrical performance.