The zinc-halogen batteries that replace the vanadium-based or manganese-based cathodes used in aqueous zinc-ion batteries with halogen redox reactions have gradually come into view in recent years, but the poor surface stability of anodic zinc as well as uncontrollable accumulation of by-products in aqueous electrolyte limited their further development. Here, the eutectic electrolyte based on N-methylacetamide is proposed for zinc–iodine battery, in which Zn²⁺ shows a unique double-shell solvated structure with a tighter anhydrous inner layer. In the case that the amount of free H₂O is effectively restricted, I⁻ has a looser solvated environment and the formation of I₃⁻ as an intermediate product during I₂ reduction is inhibited. While adopting activated carbon-coated carbon fiber cloth as adsorptive cathode, the zinc–iodine battery enables capacity retention of 98.7% after 5000 cycles with the single-cycled coulombic efficiency close to 100%. In addition, the evolutions of both aqueous and eutectic electrolytes during the operation are successfully collected in the open device assembled from cuvette by in situ UV-vis absorption spectra, further highlighting the advantages of such a eutectic electrolyte.