The present invention relates to a method for producing a 3- to 3.5-valent vanadium solution from a 4-valent vanadium solution by a catalytic reaction in the presence of a reducing agent, which generates a gas product during oxidation; a method for producing an electrolyte for a vanadium redox flow battery; and an apparatus for producing a liquid electrolyte for a vanadium redox flow battery. The present invention is characterized in that when a 3- to 3.5-valent vanadium electrolyte is produced from a 4-valent vanadium electrolyte by a catalytic reaction in the presence of a reducing agent, which generates a gas product during oxidation, the gas product produced in the catalytic reaction is captured with inert gas bubbles, which are carrier gases, and is removed from the reaction solution of the catalytic reaction by gas-liquid phase separation, thereby accelerating the catalytic reaction towards the forward reaction.