As an electrochemical inert cation, Mg²⁺ has an ionic radius (0.072 nm) similar to that of Li⁺ (0.076 nm), which is widely used to replace Li⁺ in Li?rich layered oxides (LLOs) materials. However, the influence of Mg²⁺ on the crystal structure of LLOs materials is still controversial. In this work, the Mg?doped Li?rich cathode materials Li_1.2-x Mg _x Mn_0.54Ni_0.13Co_0.13O₂ were synthesized by a sol?gel and high?temperature calcination method. The crystal structure, and valence state of elements in synthesized materials were systematically studied via X?ray diffraction, and X?ray photoelectron spectroscopy. These results indicated that Mg²⁺ doping can increase the cell parameters of LLOs materials. At the same time, compared with Li_1.2Mn_0.54Ni_0.13Co_0.13O₂, Mg?doping can effectively improve the electrochemical performance of LLOs materials. After optimization, the Mg?0.03 sample exhibits anomalous electrochemical performance, that is, the initial discharge?specific capacity is 291.9 mA?h/g and the initial coulomb efficiency is 78.40%.