NOVELTY - Preparing electrolyte for vanadium redox flow battery using vanadium oxide, comprises preparing redox flow battery electrolyte with vanadium having oxidation state of +3.5, which can be used as cathode and anode electrolyte, by mixing electrolytes having vanadium of different oxidation states. The method comprises e.g. mixing vanadium oxide and acid solution to obtain first electrolytic solution with vanadium oxidation state of +3.5 to 5; mixing vanadium oxide and acid solution to obtain second electrolytic solution with vanadium oxidation state of +2 to +3.5 and mixing electrolytic solutions.    USE - The method is useful for preparing electrolyte for vanadium redox flow battery (claimed).    ADVANTAGE - The electrolyte cane used for both cathode and anode; and provides maximum charging and discharging efficiency. The method involves the oxidation number measurement, which does not require expensive separate electrodes or color conversion reagents and can be performed by real-time measurement in a short time using a standard electrolytic solution with intermediate oxidation state; is cost-effective and suitable for high-capacity energy storage system.    DETAILED DESCRIPTION - Preparing electrolyte for vanadium redox flow battery using vanadium oxide, comprises preparing redox flow battery electrolyte with vanadium having oxidation state of +3.5, which can be used as cathode and anode electrolyte, by mixing electrolytes having vanadium of different oxidation states. The method comprises mixing vanadium oxide and acid solution to obtain first electrolytic solution with vanadium having oxidation state of +3.5 to 5; mixing vanadium oxide and acid solution to obtain second electrolytic solution with vanadium having oxidation state of +2 to +3.5; mixing first and second electrolytic solutions to obtain mixed electrolytic solution; testing oxidation of mixed electrolytic solution as electrolyte of anode and cathode of vanadium redox flow battery using standard electrolytic solution with intermediate oxidation state; comparing oxidation numbers of mixed electrolytic solution and standard electrolytic solution until oxidation numbers of mixed electrolytic solution and standard electrolytic solution are equal; processing the mixed electrolytic solution supplied to the anode and cathode stack; and supplying electrical energy to stack for separating anolyte and catholyte from mixed electrolytic solution.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic flow chart of preparation of electrolyte for vanadium redox flow battery using vanadium oxide (Drawing includes non-English language text).