A preparation method for a high-purity ionic liquid is disclosed, which orderly comprises five steps of metathetical reaction, ion exchange for removing halide ions, ion exchange for removing potassium ions, acid-alkali neutralization and solvent recovery. The method specifically comprises the following steps of: performing stirring reaction on the haloid of a target cation and potassium hydroxide for 2-24 hours in ethanol, and filtering to remove potassium halide, so as to obtain an alkaline liquid containing the target cation; adding water to dilute, and removing the residual halide ions and potassium ions through an anion exchange unit and a cation exchange unit orderly; and performing neutralization reaction for 2-6 hours with the conjugate acid of a target anion to obtain ionic liquid solution, filtering to remove solid insoluble substances, evaporating to recover a solvent, and drying to obtain the high-purity ionic liquid. Showed in the figure, the method disclosed by the invention is simple in technical process, good in the quality of product, low in cost, capable of unifying a plurality of ionic liquids in common technology, and with great industrialization prospect.