Sodium superionic conductors (NASICONs) show significant promise for application in the development of cathodes for sodium-ion batteries (SIBs). However, it remains a major challenge to develop the desired multi-electron reaction cathode with a high specific capacity and energy density. Herein, we report a novel NASICON-type Na3.5MnCr0.5Ti0.5(PO4)3 cathode obtained by combining electrospinning and stepwise sintering processes. This cathode exhibits a high discharge capacity of 160.4 mAh g-1 and operates at a considerable medium voltage of 3.2 V. The Na3.5MnCr0.5Ti0.5(PO4)3 cathode undergoes a multi-electron redox reaction involving the Cr3+/4+ (4.40/4.31 V vs. Na/Na+), Mn3+/4+ (4.18/4.03 V), Mn2+/3+ (3.74/3.41 V), and Ti3+/4+ (2.04/2.14 V) redox couples. This redox reaction enables a three-electron transfer during the Na+ intercalation/de-intercalation processes. As a result, the Na3.5MnCr0.5Ti0.5(PO4)3 demonstrates a significant enhancement in energy density, surpassing other recently reported SIB cathodes. The highly reversible structure evolution and small volume changes during cycling were demonstrated with in-situ X-ray diffraction, ensuring outstanding cyclability with 77% capacity retention after 500 cycles. Furthermore, a NMCTP@C//Sb@C full battery was fabricated, which delivered a high energy density of 421 Wh kg-1 and exhibited good cyclability with 75.7% capacity retention after 100 cycles. The rational design of composition regulation with multi-metal ion substitution holds the potential to unlock new possibilities in achieving high-performance SIBs. A novel NASICON-structured Na3.5MnCr0.5Ti0.5(PO4)3 nanofiber was successfully designed and prepared. This nanofiber was employed to research the multi-electron reaction and the resulting structural evolution in SIBs. The optimal Na-migration pathway has also been investigated by DFT computations. A full SIB battery was fabricated and delivered a high energy density (421 Wh kg-1) and cyclability (75.7% after 100 cycles at 100 mA g-1).