A series of sulfonated poly(aryl ether sulfone)s proton exchange membranes (4SPAES-x) with four flexible side chains in repeated units were prepared by bromination and graft sulfonation. Their chemical structure was characterized by 1H NMR, and their phase morphology was analyzed by atomic force microscope. It was confirmed that the membranes had a good hydrophilic/hydrophobic phase separation morphology. The ion exchange capacity of 4SPAES-x membrane is 1.12~1.74 mmol/g, and their water absorption, swelling rate and proton conductivity are 11%~32%, 7%~22% and 21~86 mS/cm at 30 ℃, respectively, which increases with the increase of the sulfonation ratio. The maximum energy efficiency of the vanadium flow single cell assembled with 4SPAES-25 membrane is 83.3% at a current density of 40 mA/cm2, which is higher than the 81.5% of Nafion 115. In addition, the efficiency of the single cell also has good cycle stability.