The invention discloses a composite electrolyte taking an azo substance as an additive and a preparation method thereof. The composite electrolyte is composed of a blank electrolyte and an electrolyte additive, wherein the blank electrolyte is a KOH solution, and the electrolyte additive is an azo substance. The concentration of the KOH in the composite electrolyte is 1-6mol/L, and the concentration of the azo substance is 1-10mmol/L. According to the invention, the method is easy to operate, raw materials are cheap and easy to get, the ionic conductivity of the electrolyte can be improved by taking the azo substance as the electrolyte additive, the internal resistance of the electrolyte is reduced, the electrochemical performance of a supercapacitor is significantly improved, and the composite electrolyte is enabled to have the advantages of high power density, high energy density, good cycle life and the like.