Rule of electrolyte volume change was studied when the all vanadium redox flow battery was running stably with a cation exchange membrane. Electric transfer of ions will make the volume change linearly with the changing of charge-discharge capacity, that is, the positive electrolyte will be less and the negative electrolyte will be more during the charge process, which will be reversed in the discharge process. The direction of net infiltration of vanadium ions is from the negtive electrolyte to the positive electrolyte, which will make the volume of positive electrolyte larger in a couple of cycles. The direction of net water transfer is the same as the vanadium ions.

Key words: all vanadium redox flow battery, electrolyte, rate of volume change, ionic mobility