Driven by global demand for new energy, Li-ion batteries (LIBs) have developed rapidly due to their competitive performance. Although LIBs show the advantages of high capacity and good cycling stability, their disadvantages such as uneven distribution of lithium resources are gradually exposed. Therefore, with abundant reserves, Na-ion batteries (NIB) have become one of the most promising solutions to make up for the deficiency of Li-ion battery. The NIBs layered oxide cathodes have the most potential applications of cathode material due to their high specific capacity (167 mAh·g ^–1in 2.4–4.3 V) and simple synthesis method. However, improving the cycling stability of layered cathode materials is one of the keys to their large-scale industrialization. To develop high capacity and cycling stability cathode materials, the Mg ²⁺is substituted for Ni ²⁺in NaNi _0.4Cu _0.1Mn _0.4Ti _0.1O ₂(NCMT), thereby obtaining a NaNi _0.35Mg _0.05Cu _0.1Mn _0.4Ti _0.1O ₂(NCMT-Mg) cathode material. The NCMT-Mg has a high reversible specific capacity of 165 mAh·g ^–1in a voltage window of 2.4–4.3 V. The reversible specific capacity of about 110 mAh·g ^–1at 0.1 C after 350 cycles with a capacity retention of 67.3% is about 13% higher than the counterpart of NCMT. The irreversible reaction is suppressed from P'3 phase to X phase for NCMT. The ex-XRD spectrometers further prove that the NCMT-Mg shows a P3 and X mixed phase after being initially charged to 4.3 V, but the NCMT shows an X phase. The irreversible phase transition is suppressed to increase the cycling stability. The inactive Mg ²⁺replaces Ni ²⁺, reducing the charge compensation and stabilizing the structure, the inactive Mg ²⁺can activate the charge compensation of Ni ²⁺/Cu ²⁺. The electrochemical activity increases from 77% to 86%. The high capacity and excellent cycling stability prove that the NCMT-Mg structure remains intact after various current rates have been tested. The long cycling stability mechanism is further systematically studied by using various technologies. The present work will provide an important reference for developing high-performance Na-ion cathode materials.