NOVELTY - Preparing vanadyl sulfate electrolyte, comprises: adding alkali metal or alkaline earth metal oxide or hydroxide to vanadium slag, and adding a reducing agent to the mixture and then mixing; adding tributyl phosphate, sulfonated kerosene and an extracting agent to the mixture and then extracting to obtain vanadium organic phase, and separating vanadyl sulfate solution from the extracting liquid; adding pure ammonia water to the solution, adding an organic reducing agent to the mixture and then extracting, and adding sulfuric acid solution to the extract; and distilling the extract.    USE - The method is useful for preparing vanadyl sulfate electrolyte of a flow battery (claimed), preferably vanadium flow battery.    ADVANTAGE - The method ensures simple and cost-effective preparation of the vanadyl sulfate electrolyte with high yield.    DETAILED DESCRIPTION - Preparing vanadyl sulfate electrolyte, comprises: (1) adding alkali metal or alkaline earth metal oxide or hydroxide to vanadium slag, adjusting pH value of the mixture to 2.5-3, and adding a trivalent iron reducing agent, an organic reducing agent and an inorganic reducing agent to the mixture for 1-2.5 times and then mixing, where the alkali metal or alkaline earth metal oxide or hydroxide is sodium hydroxide, calcium hydroxide, potassium hydroxide, calcium oxide, sodium oxide, magnesium oxide or potassium oxide; (2) adding tributyl phosphate, sulfonated kerosene and an extracting agent to the mixture at a weight ratio of 10-35:5-15:50-85 and then extracting for 5-15 minutes, allowing the mixture to stand for 3-8 minutes, washing the mixture for 3 minutes using 200 g/l of sulfuric acid to obtain a vanadium organic phase, further extracting the vanadium organic phase for 4-8 minutes, and separating vanadyl sulfate solution from the extracting liquid; (3) adding pure ammonia water to the separated vanadyl sulfate solution and then adjusting the pH value to 2.5-3, adding an organic reducing agent to the mixture and then mixing, extracting the mixture for 5-10 minutes, adding less than 5 g/l of sulfuric acid solution to the extract and then washing for 3 minutes, and further extracting the mixture for 4-8 minutes; and (4) distilling the extract to obtain the vanadyl sulfate electrolyte. The vanadyl sulfate electrolyte has a concentration of less than 0.8 mole/l, a pH value of less than 1 and an impurity content of 0.008-0.05.