The research of sodium-ion batteries (SIBs) is of great significance for development of new energy and energy storage methods. As cathode material, P2-type layered oxide material Na_2/3Ni_1/3Mn_2/3O₂ has attracted wide attention due to its excellent capacity and high working voltage. However, it suffers from undesired P2-O2 phase transition, which leads to a drastic change in volume and rapid capacity decay. Here, a P2-Na_0.67Ni_0.18Cu_0.10Mg_0.05Mn_0.67O₂ (NCMM-10-05) cathode was synthesized through solid-state method with synergetic substitution of Cu and Mg. The results indicated that the incorporation of Cu and Mg suppressed irreversible P2-O2 phase transition when charging to high voltage and initialized OP4 phase formation, which improved reversible stability of structure. Thus the as-obtained material exhibited excellent electrochemical performance, which delivered an initial discharge capacity of 113 mAh·g^-1 in the voltage range of 2.00-4.35 V (vs. Na⁺/Na), a reversible capacity of 64.1 mAh·g^-1 at 8C (1C=100 mA·g^-1), and a capacity retention of 88.9% after 200 cycles at 1C. The effect of Cu and Mg synergetic substitution on the structure and electrochemical properties of P2-type layered oxides was explored, and the specific roles played by Cu and Mg in the structural evolution were further investigated by in situ X-ray diffraction (XRD) analysis and density functional theory (DFT) calculations. This work provides a new insight into the rational design of highly stable cathode materials with rapid Na⁺ transport capability for SIBs.