The invention relates to a preparation method and device of a high-voltage lithium battery electrolyte, and the method comprises the following steps: firstly, uniformly mixing a carbonate electrolytesystem, sulfolane, ethyl vinyl sulfone, ethyl isopropyl sulfone and acrylonitrile, and carrying out heating reflux reaction for 16-20 h to obtain an initial electrolyte; adding lithium salt into the initial electrolyte, and carrying out a heating reflux reaction for 25-30 h to obtain a secondary electrolyte; cooling the secondary electrolyte to room temperature, and adding a neutralization reaction system to obtain a tertiary electrolyte; adding a non-aqueous cosolvent into the tertiary electrolyte, and performing uniform stirring to react for 6-8 h, thereby obtaining the high-voltage lithiumbattery electrolyte. According to the high-voltage battery electrolyte prepared by the embodiment of the invention, the continuous oxidative decomposition of a carbonate electrolyte system is effectively reduced by adding sulfone substances; and the non-aqueous cosolvent is added, so that the viscosity of sulfone substances is reduced, the oxidation capacity is maintained, and the carbonate solvent can also have a good application effect in a high-voltage system.