Electrospinning has been employed to fabricate carbonaceous materials with larger surface areas for vanadium redox flow batteries. However, the woven carbon nanofibers prepared with conventional electrospinning methods are plagued by the low porosity and poor permeability, thereby causing a significant mass-transport resistance during the operation of batteries. To tackle this problem, we report a novel method by self-assembling porous carbon fibers into large bundles to form electrodes. This electrode is fabricated by electrospinning polyacrylonitrile and polystyrene binary solutions. Instead of forming single fibers, the individual fibers are self-assembled into fiber bundles by properly managing the viscosity of the precursor solution. The formation of large fiber bundles significantly enlarges the pore size while retaining large specific surface areas. The single cell with the as-prepared electrodes achieves an energy efficiency of 87.7% at a current density of 100 mA cm−2, which is 15.2% higher than that of the single cell with conventional electrospinning electrodes. The energy efficiency still maintains over 80% at 200 mA cm−2. More importantly, the discharge capacity and electrolyte utilization are nearly doubled. All these results demonstrate that this electrode preparation method is effective to improve the mass transport properties of traditional electrospun electrodes in vanadium redox flow batteries.