The invention relates to an online detecting method and system for the concentration of all ions of an all-vanadium redox flow battery. The method includes the following steps that A, the refractive index of a positive electrode electrolyte and the refractive index of a negative electrode electrolyte are measured in an online mode; B, the open-circuit voltage of the positive electrode electrolyte and the open-circuit voltage of the negative electrode electrolyte are measured in an online mode; C, the volume of the positive electrode electrolyte and the volume of the negative electrode electrolyte are measured in an online mode; D, the concentration of all the ions in the all-vanadium redox flow battery is acquired in an online mode according to the online measurement results. By the adoption of the steps, the concentration of all the ions of the vanadium redox flow battery can be detected. According to the online detecting method for the concentration of all the ions of the all-vanadium redox flow battery, the concentration of all the ions at the positive electrode and the negative electrode of the vanadium battery can be detected in real time in an online mode, and conditions are provided for solving the problem of electrolyte imbalance and studying the electrochemical process.