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In recent years, ternary nickel-rich layered oxides have gradually replaced traditional binary cathode materials in the lithium-ion battery market due to their advantages of high energy density and environmental protection. However, their structural instability of cathode materials has seriously affected the cycle performance of the battery. In order to optimize the internal structure of LiNi0.5Co0.2Mn0.3O2, the first-principle calculation was carried out in advance. The energy band density, state density and volume changes were obtained by the simulation calculation. Meanwhile, the electrochemical performance of modified LiNi0.5Co0.2Mn0.3O2 prepared by in-situ doping Na and Cl by wet grinding solid phase method was investigated. The results show that the impedance, capacity and capacity retention data are very compatible with the simulation results. The modified sample exhibits a good cyclic stability with the capacity retention of 80.91% after 80 cycles at 1 C, which is higher than that of 70.00% for commercial LiNi0.5Co0.2Mn0.3O2. The scanning electron microscope characterization indicates that the co-doping of Na and Cl can effectively alleviate the corrosion of of LiNi0.5Co0.2Mn0.3O2 surface by electrolyte. Therefore, Na and Cl doping can effectively optimize the internal structure of LiNi0.5Co0.2Mn0.3O2 and improve its electrochemical performance. The combination of simulation calculation and experiment provides a new approach for the modification of ternary cathode materials.