Mn3O4/MWCNTs (multi-walled carbon nanotubes) composite fabricated by a simple solvothermal method was developed as electrochemical catalyst of V(IV)/V(V) redox couple for vanadium redox flow batteries. The electrochemical activity of V(IV)/V(V) redox couple can be enhanced by the electrochemical catalysts (Mn3O4, MWCNTs, Mn3O4/MWCNTs), and the electrocatalytic kinetics of the redox reactions were in the order of Mn3O4/MWCNTs>MWCNTs>Mn3O4. The cell using Mn3O4/MWCNTs composite as electrochemical catalyst was assembled and the charge-discharge performance was evaluated. Compared with the pristine cell, the cell using positive graphite felt modified by Mn3O4/MWCNTs had lower electrochemical polarization, larger discharge capacity and energy efficiency. The average energy efficiency of the cell using modified positive electrode for 50 cycles was 84.65%, 3.73% higher than that of the pristine cell. The superior electrocatalytic performance of Mn3O4/MWCNTs composite was mainly due to the effective mixed conducting network, facilitating the electron transport and ion diffusion in the electrode/electrolyte interface.