According to the method for preparing an electrolyte for a vanadium redox flow battery, one electrolyte can be used as both the positive electrolyte and the negative electrolyte, by preparing an electrolyte having a median oxidation number of electrolytes used for the positive electrode and the negative electrode of the vanadium redox flow battery. Particularly, since the mixed electrolyte having the median oxidation number is separated into the same amounts of positive electrolyte and the negative electrolyte at the time of charging and discharging, the maximum charging and discharging effect based on the supplied capacitance can be obtained.