The invention discloses a preparation method of an electrolyte for an all-vanadium redox-flow battery and particularly provides a preparation method of an electrolyte for an all-vanadium redox-flow battery which is high in concentration, good in stability and low in production cost. The problem that vanadium batteries in the prior art are low in electrolyte concentration, poor in stability, high in preparation cost and the like is solved. The preparation method of the electrolyte for the all-vanadium redox-flow battery comprises the following steps of using an oxalic acid and other reducing agents to reduce a part of vanadium pentoxide, then adding a stabilizing agent, arranging the agent in an electrolytic cell, performing constant-current electrolysis to obtain a trivalent vanadium and tetravalent vanadium mixed electrolyte, and the total vanadium concentration of the electrolyte for the all-vanadium redox-flow battery is 1.0 mol/L-5.0 mol/L. The preparation method of the electrolyte has the advantages that the process is simple, reaction conditions are easily controlled, the cost is remarkably reduced, the concentration performance of the obtained electrolyte is stable, the range of usage temperature is wide, industrial preparation is easy to achieve, and the like.