A system and method for preparing a vanadium battery high-purity electrolyte, comprising preparing a low-valence vanadium oxide with a valence of 3.5 with liquid phase hydrolysis and fluidization reduction with vanadium oxytrichloride, adding clean water and sulfuric acid for dissolution, and further performing ultraviolet activation to obtain the vanadium electrolyte, for use in an all-vanadium redox flow battery stack. The high-temperature tail gas in the reduction fluidized bed is combusted for preheating the vanadium powder material, to recover the sensible heat and latent heat of the high-temperature tail gas, and the sensible heat of the reduction product is recovered through heat transfer between the reduction product and the fluidized nitrogen gas. An internal member is arranged in the reduction fluidized bed to realize the precise regulation of the valence state of the reduction product, and ultraviolet is used to activate the vanadium ions, improving the activity of the electrolyte.