High-voltage sodium-ion batteries (SIBs) hold promise for energy storage, but unstable electrode/electrolyte interphases (EEI) due to excessive solvent decomposition under high voltage (>4.2 V) hinder their practical viability. Herein, a dual-additive electrolyte system is rationally designed via synergistic integration of theoretical calculations and experimental optimization to stabilize the EEI. Density functional theory calculations identify fluoroethylene carbonate (FEC) and 1,3,5-tri-2-propenyl-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione (TAIC) as functional additives. Specifically, FEC lowers the highest occupied molecular orbital (HOMO) energy level of the electrolyte through electron-withdrawing effects, thereby enhancing oxidative stability, while TAIC prioritizes oxidation-reduction to alleviate solvent decomposition because of its low lowest unoccupied molecular orbital energy level (-0.72 eV) and high HOMO energy level (-7.53 eV). Molecular dynamics simulations reveal that both additives could reduce the solvent coordination number from 4.35 to 4.26, further promoting the formation of an additive-derived EEI. The full cell with optimized electrolyte (5 wt.% FEC, 0.2 wt.% TAIC) delivers a high specific capacity of 120.56 mAh g−1 at 0.02 A g−1 and 101.59 mAh g−1 at 0.1 A g−1 under 4.35 V. Moreover, XPS and TEM characterizations revealed that the dual-additive system induces the formation of a Na3N/NaF-enriched composite EEI, thereby synergistically enhancing transport of Na+ and mechanical strength.