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Na-ion batteries are promising for large-scale energy storage applications, but few cathode materials can be practically used because of the significant difficulty in synthesizing an electrode material with superior comprehensive performance. Herein, an effective strategy based on synergetic contributions of rationally selected metal ions is applied to design layered oxides with excellent electrochemical performances. The power of this strategy is demonstrated by the superior properties of as-obtained NaFe0.45Co0.5Mg0.05O2 with 139.9 mA h g−1 of reversible capacity, 3.1 V of average voltage, 96.6% of initial Coulombic efficiency, and 73.9 mA h g−1 of capacity at 10 C rate, which benefit from the synergetic effect of Fe3+ (high redox potential), Co3+ (good kinetics), and inactive Mg2+ with compatible radii (stabilizing structure). Moreover, it is clarified that the superior property is not the simple superposition of performance for layered oxides with single metal ions. With the assistance of density functional theory calculations, it is evidenced that the wide capacity range (>70%) of prismatic Na+-occupied sites during sodiation/desodiation is responsible for its high rate performance. This rational strategy of designing high-performance cathodes based on the synergetic effect of various metal ions might be a powerful step forward in the development of new Na-ion-insertion cathodes.