The invention relates to the technical field of electrolytes and particularly to an ultrahigh-voltage electrolyte solute and a preparation method thereof. The ultrahigh-voltage electrolyte solute is composed of mannitol, boric acid, dodecanedioic acid and ethylene glycol in a weight ratio of 5-15: 10-20: 5-15: 50-80. Used in an electrolyte, the electrolyte solute has sparking voltage as high as 550V, electric conductivity of 1.2ms/cm, good electrochemical properties, high electric conductivity and ultrahigh voltage, and can be used for manufacture of ultrahigh-voltage aluminum electrolytic capacitors.