Sb3+ ions are introduced into the negative electrolyte of vanadium redox flow batteries (VRFB), and their influence on the electrochemical performance of VRFB are investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The results show that the electrochemical activity and sluggish kinetics of V(II)/V(III) redox couple can be improved by the addition of Sb3+ ions, and the optimal concentration of Sb3+ ions is found to be 5mM. Meanwhile, Sb3+ ions can lead to an increase of the diffusion coefficient of V(III) species and a decrease of charge transfer resistance. Moreover, the VRFB cell using negative electrolyte with Sb3+ ions exhibits excellent cycling stability and high average energy efficiency, especially under high power operation. The energy efficiency (67.1%) of the VRFB employing electrolyte with 5mM Sb3+ ions is increased by 9.6% at a current density of 120 mA·cm−2, compared to the pristine one (57.5%). The improved electrochemical performance should be ascribed to the prominent catalytic effect of Sb particles, which are simultaneously electrodeposited onto the surface of graphite felts during operation of the flow cell and facilitate charge transfer process.