Abstract：Manganese-based NASICON cathodes enable high-energy-density sodium-ion batteries via multi-electron Mn^2+/3+/4+ redox and fast Na⁺ diffusion. However,they suffer from Jahn-Teller distortion（Mn³⁺ ）, causing structural degradation and compromised rate/cycling stability. Herein, a medium-entropy engineered Na₃Mn_2/3Ti_2/3 V_2/3(PO₄)₃ cathode material, synthesized via a scalable ball-milling method is reported, which integrates the Ti^3+/4+,V^3+/4 ,and Mn^2+/3+ redox couples. The NMTVP-V_2/3 achieves a reversible capacity of 178.8 mAh g^-1 at 20 mA g^-1 and an energy density of 509.6 Wh kg^-1. After cycling 1200, its capacity retains 82.24% at 200 mA g^-1. In-situ XRD reveals that near-equimolar Ti/Mn/V mixing creates a medium-entropy structure,promoting the mechanism shifted from a mixed solid-solution and two-phase reaction to dominant solid-solution reactions. Integrated with electrochemical characterization and DFT, the results indicate that the medium-entropy optimized NMTVP-V_2/3 enhances ionic transport and mitigates phase degradation. Furthermore, the NMTVP- V_2/3 ||hard carbon full cell exhibits a high capacity of 162.9 mAh g^-1 at with an outstanding rate performance. This strategy can be generalized to other polyanionic systems, presenting a novel approach for developing SIB cathodes.