The hybrid P3/P2-structured Na0.5Ni0.3Mn0.7O2 as a cathode material for sodium-ion batteries (SIBs) demonstrates high initial discharge capacity. However, the existence of an unstable P3 phase leads to a fast capacity fade that is adverse to engineering applications. Herein, the introduction of inactive magnesium to partially replace electrochemically active nickel of Na0.5Ni0.3Mn0.7O2 is proposed to reinforce the structural stability of the material. It is found that magnesium incorporation can suppress the growth of the P3 V phase, and proper substitution of magnesium for nickel can even harvest pure P2 phase material. The X-ray diffraction (XRD) pattern reveals that the prepared Na0.5Ni0.2Mg0.1Mn0.7O2 (Mg-0.1) is a structure-stable P2-type material. The fabricated Na0.5Ni0.2Mg0.1Mn0.7O2 SIB cathode exhibits acceptable initial discharge capacity, tunable rate capability, and satisfactory cycling stability. These superior electrochemical properties account for low electrochemical resistances, fast Na+ diffusion kinetics, and the extremely stable structure of the P2-Na0.5Ni0.2Mg0.1Mn0.7O2 material, which are revealed by means of electrochemical impedance spectra and ex situ XRD technologies.