The invention provides a preparation method of high-purity lithium hexafluorophosphate, which is characterized in that the characteristic that PF5 generation is facilitated due to higher solubility of hexafluorophosphate MPF6 in a mixed solvent is utilized. MPF6 is dissolved in the mixed solvent in a reaction tank; MPF6 and anhydrous HF in the mixed solvent react to generate HPF6; HPF6 cannot exist stably under a heating condition, and is decomposed to form PF5; generated PF5 and anhydrous LiF react in an absorption tank to generate LiPF6; a cooling medium is supplied into a reflux condenser tube, so that a part of evaporated anhydrous HF in the reaction tank can be condensed, and MPF6 can be dissolved continuously and converted into HPF6; and finally a high-purity lithium hexafluorophosphate product is obtained by continuous drying under LiPF6 treated inert gas.