A system and method for producing a high-purity and high-activity vanadium electrolyte, comprising converting high-purity vanadium oxytrichloride into an ammonium salt in a fluidized bed by gas phase ammoniation, then in another fluidized bed, reducing the ammonium salt into a low-valence vanadium oxide having an average vanadium valence of 3.5, adding clean water and sulfuric acid for dissolution, and further performing activation by ultrasound to obtain a 3.5-valence vanadium electrolyte which can be directly used in a new all-vanadium redox flow battery stack. The method of producing an ammonium salt containing vanadium in the fluidized bed by gas phase ammoniation is of short process and high efficiency. Precise regulation of the valence state of the reduction product is implemented by arranging an internal member in the reduction fluidized bed, and ultrasonication is used to activate the vanadium ion, thereby greatly improving the activity of the electrolyte.