The invention discloses a stable control method and system for capacity of a redox flow battery and the redox flow battery. The system comprises an acquisition unit, a first judgment unit, a second judgment unit and a control unit, wherein the acquisition unit is used for acquiring a current electrolyte capacity attenuation ratio of the redox flow battery, the first judgment unit is used for judging whether the current electrolyte capacity attenuation ratio of the redox flow battery is lower than a first preset attenuation ratio or not, the second judgment unit is connected with the first judgment unit and is used for judging whether the current electrolyte capacity attenuation ratio of the redox flow battery is lower than a second preset attenuation ratio or not when the current electrolyte capacity attenuation ratio of the redox flow battery is higher than or equal to the first preset attenuation ratio, and the control unit is connected with the second judgment unit. By the system, the capacity retention capability of the redox flow battery can be effectively improved, the cost of a capacity recovery agent is reduced, and the capacity and the performance of the redox flow battery are maintained stable for a long time.