Ni/Mn/Fe-ternary cathode is highly regarded as a potential option for sodium-ion batteries (SIBs) due to its low cost and high theoretical capacity. Nonetheless, the cycling stability was hindered by the occurrence of high voltage phase transitions. In this work, Na0.67(Ni0.1Mn0.8Fe0.1)1-xMgxO2 (NaNMF-Mgx) cathode materials with high-voltage zero-phase transitions property were successfully synthesized. And the element Mg was firstly found to have the potential to eliminate high-pressure phase transitions in Ni/Mn/Fe-ternary systems. By employing the Mg-doping strategy, there were a valuable strengthening in the occupancy of stabler Nae sites. And the calculation highlighted the excellent structural stability and conductivity of NaNMF-Mg0.04, which has the lowest thermodynamic formation energy and a narrow band gap. Moreover, ex-situ XRD and ADF-STEM certified that NaNMF-Mg0.04 cathode maintained an intact P2 phase structure during high-voltage charging process, which was different from previous reports. The “zero-phase transitions” effect enabled NaNMF-Mg0.04 cathode to express remarkable initial capacitance (119.5 mAh g-1, 0.1C) and stability (80.0 % over 200 cycles). These findings further declared that it was feasible to design novel Ni/Mn/Fe-ternary cathodes with zero-phase transitions property, which might open up a new idea for popularizing cost-effective Ni/Mn/Fe-ternary cathode material systems in the future.