Proton exchange membrane (PEM) is one of the core components of vanadium redox flow battery (VRFB). Excellent PEMs should solve the current problems of high cost and complicated synthesis process and have key properties such as high proton conductivity, low vanadium ion permeability, high mechanical strength and excellent chemical stability. Herein, a series of poly(fluorenyl ether ketone)s (PFEKs) were synthesized based on tetramethylbisphenolfluorene monomer by condensation polymerization, and the phenylmethyl groups were functionalized to bromomethyl groups followed by SN₂ nucleophilic substitution with sodium 4-hydroxybenzenesulfonate to yield a series of sulfonated poly(fluorenyl ether ketone)s (SPFEKs) with different ion exchange capacities. A series of novel low-cost PEMs were obtained by solution casting SPFEKs into membranes and acidification. The synthetic route is based on a wide and inexpensive source of raw materials, which avoids dangerous sulfonation reactions, making it easy for industrial scale-up. All of the prepared PEMs have good mechanical properties and oxidative stability, among which the SPFEK-40 membrane has the most balanced properties such as high proton conductivity, high ion selectivity, low vanadium ion permeability, and low areal resistance. The energy efficiency of the VRFB assembled with SPFEK-40 membrane was 88.2% at a current density of 80 mA/cm^₂, which is higher than that of the VRFB assembled with Nafion 212 membrane (84.8%). In addition, the discharge capacity of the VRFB assembled with SPFEK-40 membrane only decreased to 84.3% after 30 cycles, which is much higher than that of the VRFB assembled Nafion 212 membrane (66.1%).