A novel oxidized g-C3N4 (OCN)-modified Sulfonated poly(ether ether ketone) (SPEEK) composite membrane were fabricated via a solution-casting method. The physicochemical properties of composite membranes with different oxidized-C3N4 content were characterized, including water uptake, swelling ratio, ion exchange capacity, proton conductivity, vanadium ion permeability and ion selectivity. It was found that when the content of OCN incorporated into SPEEK matrix was 1%, this composite membrane showed a low vanadium ion permeability(9.09×10− 7 cm2 min−1) and high ion selectivity(6.60×104 Smincm−3). The vanadium redox flow battery (VRB) assembled with this membrane at 60mAcm−2 exhibited a higher coulombic efficiency (98%) than Nafion 117 membrane (95%). Moreover, in cycling tests VRB assembled with this composite membrane showed a more stable cycle performance compared with that of Nafion 117 membrane. Thus, the OCN-modified SPEEK composite membrane has a potential application for VRB system.