With the advantages of low cost and wide distribution of raw materials, sodium-ion batteries are considered to be the best alternative materials for lithium-ion battery cathode materials. In the P2-phase NaMnO₂ with layered structure, binary solid solution of the transition metal layer can effectively improve the electrochemical performance of the electrode material. In this study, the structural model of Na_x[Mg_0.33Mn_0.67]O₂ with Mg ion solid solution was constructed by using the Coulombic model. The first-principles calculations revealed that discharge voltage of Na_x[Mg_0.33Mn_0.67]O₂ reached 3.0 V at a sodium ion content of less than 0.67. Electronic density of states and charge population analysis showed that the solid solution of Mg motivated the anionic electrochemical activity of lattice oxygen in the P2-phase Na_x[Mg_0.33Mn_0.67]O₂, which transformed the electrochemical reaction mechanism of the system from cationic and anionic synergic redox reaction to reversible anionic redox reaction. This transformation provides a novel method for the design of electrode materials for Na ion batteries, as well as a new approach for the optimization and exploration of other ion batteries.