The invention provides a method for controlling the valence state equilibrium of electrolyte in the energy storage module of an all-vanadium liquid flow battery. An electrolyte valence state reductiondevice is connected in parallel into a negative electrolyte pipeline, the valence state of electrolyte in the energy storage module of an all-vanadium liquid flow battery is reduced by electrolysis,and the electrolysis is stopped when the valence state reaches equilibrium. According to the invention, an electrolyte valence state reduction device is connected in parallel into a negative electrolyte pipeline, there is only the need to consume electric energy, there is no need to add other substances into the energy storage module, and the pump and the pipeline of the energy storage module arefully utilized. The method is simple and convenient to use and simple to operate, and can reduce labor cost compared with other methods. Meanwhile, online operation can be carried out, and the goal ofreducing the overall valence state of electrolyte in the energy storage module is achieved without stopping the operation of the energy storage module.