O3-type NaNi_0.5Mn_0.5O₂ cathode material was modified with NH₄VO₃ solution through a one-step liquid-phase method. The effects of NH₄VO₃ solution treatment on the phase structure and electrochemical performance of NaNi_0.5Mn_0.5O₂ were investigated. X-ray diffraction reveals that the use of auxiliary solvent and secondary heat treatment induces the formation of a V₂O₃/P2/O3 multiphase structure. The electrochemical results indicate that the multi-level structure can alleviate Jahn-Teller distortion and suppress irreversible phase transitions. Besides, the in-situ reconstruction layer can hinder the formation of hydrated phases, further enhancing the environmental stability of NaNi_0.5Mn_0.5O₂. Consequently, the modified NaNi_0.5Mn_0.5O₂ cathode material achieves an excellent capacity retention of 74.9% after 1000 cycles at 5C (1C=150 mA/g), whereas the original sample only retains 42.6% of its capacity. These results indicate that NH₄VO₃ surface treatment is an effective strategy for improving the stability of NaNi_0.5Mn_0.5O₂.