A simple freeze-drying-assisted method is used to prepare various porous Mn-doped Na3V2(PO4)3/C composites. Rietveld refinement and atomic absorption spectroscopy (AAS) are applied to probe the real substitute site of Mn and demonstrate that Mn ions successfully occupy the V sites. The Mn2+/Mn3+ doping in the vanadium site of Na3V2(PO4)3 causes the lattice volume to expand with enlarged Na ion diffusion channels, which benefits Na+ transportation and increases the ionic conductivity of Na3V2(PO4)3. Furthermore, due to the charge compensation, the presence of Mn2+ in Na3V2(PO4)3 leads to the formation of V4+, which improves the intrinsic conductivity and stabilizes the NASICON framework. Through the XPS results of the electrode under the fully charged or discharged states, the small and reversible plateau at ca. 3.85 V of the Mn-doped Na3V2(PO4)3 electrode is ascribed to the activation of the Mn2+/Mn3+ redox couple. An optimized performance is achieved for Na3V1.8Mn0.2(PO4)3/C, which exhibits a capacity of 106.8 mAh g−1 at 1 C and 77.8 mAh g−1 at up to 30 C. Even more impressive, a high capacity retention of 82% was achieved at 30 C after 10000 cycles.