A system and method for preparing a high-purity vanadium electrolyte, comprising preparing a low-valence vanadium oxide with vanadium oxytrichloride by ammonium salt precipitation and fluidization reduction, and preparing the high-purity vanadium electrolyte at a low temperature by adding a sulfuric acid solution and clean water under the conditions of ultrasound-assisted dissolution and activation. Efficient utilization of heat is achieved through heat exchange between the ammonium salt and the reduction tail gas and heat exchange between the reduction product and fluidized nitrogen gas. Ammonia gas in the reduction tail gas is recovered for precipitation of vanadium to achieve the recycling of ammonia gas. An internal member is arranged in the reduction fluidized bed to realize the precise regulation of the valence state of the reduction product. Furthermore, ultrasound-assisted dissolution and activation are employed to prepare the vanadium electrolyte at a low temperature, thereby improving the activity of the electrolyte.