The poor hydrophilicity and weak activity of graphite felt (GF) electrodes hinder the efficient application of vanadium flow batteries (VFB). Therefore, enhancing the electrolyte wettability and electrochemical activity of GF is an effective way to improve the energy efficiency (EE) and power density of VFB. In this work, we report a simple and environmental friendly method to prepare phosphorus-doped carbon nitride modified GF (P-C3N4@GF) electrode, in which the P-C3N4 electrocatalysts are homogeneously distributed on the surface of GF. Owing to the excellent electrolyte wettability and notable electrocatalytic activity, the as-prepared P-C3N4@GF electrode exhibits significantly enhanced electrochemical activity towards VO2+/VO2 + and V2+/V3+ redox couples. As a result, the VFB single cell with P-C3N4@GF electrode demonstrates outstanding rate performance at current densities of 50–300 mA cm−2. Specifically, a maximum power density of 616.2 mW cm−2 is achieved. Moreover, the P-C3N4@GF electrode exhibits superior stability and durability at a high current density of 150 mA cm−2. Based on the above results, the possible catalysis mechanisms of P-C3N4@GF towards positive and negative reactions of VFB are discussed.