A series of sulfonated poly(arylene ether sulfone) block copolymers with aliphatic chains (SPAES-LA) to lend structural flexibility in the polymer backbone have been synthesized to prepare proton exchange membranes (PEMs) showing improved electrochemical performance and dimensional/oxidative stabilities. The SPAES-LAs, bearing different hydrophilic/hydrophobic segment lengths, are prepared via polycondensation and sulfonation reactions. The sulfonation reaction occurs in specific fluorenylidene units by using chlorosulfonic acid. The SPAES-LA membrane, fabricated by solvent casting method, exhibits remarkable dimensional/thermal stabilities. Moreover, proton conductivity of as-prepared SPAES-LA membranes demonstrates significant improvement with expansion of ion clusters which is due to the increased hydrophilic volume ratio. In particular, the SPAES-LA-X12Y28 membrane exhibited heightened proton conductivity of 158.4 mS cm⁻¹ as well as suitable dimensional stability and durability towards radical oxidation, due to an effective well-defined hydrophilic-hydrophobic interface. Furthermore, H₂/O₂ fuel cell performance using SPAES-LA-X12Y28 membrane achieves a maximum power density of 232.02 mW cm⁻², a result which points out that SPAES-LA membranes show great potential for applications of polymer electrolyte membrane.