The invention relates to a method for on-line monitoring of the concentration of vanadium ions in an all-vanadium redox flow battery, which comprises the following steps: 1) Potential monitoring: installing a reference at the liquid inlets of the positive and negative liquid storage tanks of the all-vanadium redox flow battery The electrode and working electrode, the potential tester monitors the change of the positive electrode potential and the negative electrode potential of the all-vanadium redox flow battery in real time through the reference electrode and the working electrode connected with it. From the obtained test potential, the battery’s potential can be obtained according to the relevant potential calculation formula. Charge and discharge state; 2) Vanadium ion concentration monitoring: adopt potentiometric titration method, use titrant, and perform potentiometric titration by potentiometric titrator to obtain vanadium ion concentration. The advantages of the present invention are that the monitoring method is simple, convenient, easy to operate and control, has high accuracy, and will not pollute the electrolyte of the battery; not only the charging and discharging state of the battery can be monitored online, but also the positive and negative poles of the battery can be monitored regularly Changes in the concentration of vanadium ions in the electrolyte, and monitor whether the charge and discharge states of the positive and negative electrodes are consistent.