The Na+ superionic conductor (NASICON)-type Na3V2(PO4)(3) cathodes have attracted extensive interest due to their high structural stability and fast Na+ mobility. However, the substitution of vanadium with low-cost active elements remains imperative due to high cost of vanadium, to further boost its application feasibility. Herein, a novel ternary NASICON-type Na4VMn0.5Fe0.5(PO4)(3)/C cathode is designed, which integrates the advantages of large reversible capacity, high voltage, and good stability. The as-obtained Na4VMn0.5Fe0.5(PO4)(3)/C composite can deliver an excellent rate capacity of 96 m Ah g(-1) at 20 C and decent cycling durability of 94% after 3000 cycles at 20 C, which is superior to that of Na4VFe(PO4)(3)/C and Na4VMn(PO4)(3)/C. The synergetic contributions of multimetal ions and facilitated Na+ migration of the Na4VMn0.5Fe0.5(PO4)(3)/C cathode are confirmed by the first-principles calculations. The processive reduction/oxidation involved in Fe2+/Fe3+, Mn2+/Mn3+, V3+/V4+/V5+ redox couples are also revealed upon the charging/discharging process by ex situ soft X-ray absorption spectroscopy. The reversible structure evolution and small volume change during the electrochemical reaction is demonstrated by in situ X-ray diffraction characterization. The rational design of NASICON-type cathodes by regulating composition with substitution of multimetal ions can provide new perspectives for high-performance Na-ion batteries.