Under the capacity increase of ocean, solar and wind power, the energy storage technology has been developed to regulate the power of renewable energy and enhance the stability of power network in the past decades. A system model of all vanadium redox flow battery (VRFB) is established including the electric subsystem and hydraulic subsystem, and the accuracy and reliability of this system model are validated. The variation of pump efficiency with flow rate is considered in the proposed system model, which makes the predicted system efficiency and coulomb efficiency more accurate. A new control strategy of variable flow rate is proposed by giving a limitation region of the flow rate. In this new strategy, the coefficients of system model can be determined and adjusted according to the VRFB operating parameters, which can guarantee high system efficiency and coulomb efficiency simultaneously. In application of this new control strategy, the system efficiency and coulomb efficiency under variable flow rate strategy are always higher than that under constant flow rate, with 3.34% increasing for system efficiency and 3.84% increasing for coulomb efficiency by average.