The invention relates to the field of a redox flow battery, in particular to a method for reducing battery attenuation of an all-vanadium redox flow battery and an ion permeability test device. The method comprises the steps of testing dynamic permeation ratio of a vanadium ion in each valent state by employing the ion permeability test device; controlling ionic concentration ratio of a positive electrode and a negative electrode in the all-vanadium redox flow battery to be inversely proportional to the vanadium ion dynamic permeation ratio of a positive electrode and a negative electrode in an all-vanadium redox flow battery system; controlling the product of the vanadium ion concentration ratio of the positive electrode and the volume of the positive electrode in the all-vanadium redox flow battery to be equal to the product of the vanadium ion concentration ratio of the negative electrode and the volume of the negative electrode in the all-vanadium redox flow battery. With the adoption of an all-vanadium redox flow battery electrolyte prepared by the method, the problem of battery attenuation of the battery can be solved, and the electrolyte utilization ratio is improved. The method is simple and easy to operate, and the post maintenance cost of the vanadium battery can be greatly reduced.