Lithium-rich manganese-based cathode materials xLi₂MnO₃·(1−x) LiTMO₂ (TM=Ni,Co or Mn) are highly promising cathode materials for lithium-ion batteries due to their advantages such as high specific capacity and high voltage. However, problems like low initial Coulombic efficiency, rapid capacity and voltage decay limit their commercial applications. In this study, it is proposed to modify the lithium-rich manganese-based oxide Li_1.2Mn_0.6Ni_0.2O₂ through the co-doping of anions and cations of Ti⁴⁺ and ${\mathrm{P}\mathrm{O}}_4^{3-}$. On the one hand, Ti⁴⁺ doping can optimize the Li⁺ diffusion pathway and accelerate the diffusion rate of Li⁺, thereby improving the cycling performance and rate performance of the material. On the other hand, ${\mathrm{P}\mathrm{O}}_4^{3-}$ doping can change the electronic structure of the material, weaken the covalency of the transition metal-oxygen (TM—O) bond, inhibit the irreversible loss of lattice oxygen, and stabilize the material structure. The characterization results show that the co-doping causes the expansion of the material lattice and increases the diffusion coefficient of Li⁺. Electrochemical tests indicate that the modified material can achieve a capacity retention rate of 89.5% after 400 cycles at a rate of 1 C, which is significantly higher than that of the undoped original material. At a high rate of 5 C, the specific capacity can still reach 139.5 mAh·g⁻¹, demonstrating excellent rate performance. This study provides an effective strategy of anion-cation co-doping for the modification of lithium-rich manganese-based cathode materials for high-performance lithium-ion batteries.