The present invention provides a working electrolyte for a slender-type high-voltage aluminum electrolyte capacitor. The working electrolyte comprises a solvent, solutes and additives, wherein the solutes is composed of a main solute occupying 5%-30% the weight of the electrolyte and a subsidiary solute occupying 1%-10% the total weight of the electrolyte, the main solute is selected from aliphatic series carboxylic acids whose carbon number is 25 or 26 on main chains, and the subsidiary solute is selected from ammonium sebate, ammonium azelate, ammonium adipate and ammonium benzoate. With adoption of the electrolyte provided by the present invention, saturated vapor pressure is improved, a hydrogen removal capability is promoted, high-temperature stability is raised, the working electrolyte can be applied to the slender-type high-voltage aluminum electrolyte capacitor and can endure high temperature of 115 DEG C for 5000 hours.