A new series of sulfonated poly(arylene ether sulfone) copolymers containing phenyl side groups with different sulfonation degrees (SPAES-PQ-xx) were successfully synthesized through direct aromatic nucleophilic substitution polycondensation of 3,3′-disulfonate-4,4′-dichlorodiphenylsulfone, 4,4-dichlorodiphenylsulfone and phenylhydroquinone (PQ). The degree of sulfonation was realized by controlling the quantity of the sulfonated monomer. The chemical and phase-separated structures of copolymers were characterized by proton nuclear magnetic resonance spectroscopy and atomic force microscopy. The properties of flexible and transparent SPAES membranes were evaluated. Compared to the hydroquinone-based SPAES membrane (SPAES-HQ-40), the effect of the phenyl side group on the water absorption properties, swelling properties and proton conductivity of SPAES-PQ-xx membranes was explored. The results demonstrated that the introduction of phenyl side groups could result in better dimensional, thermal and oxidative stability compared with those of the SPAES-HQ-40 membrane. In addition, the proton conductivity of SPAES-PQ-40 is up to 0.096 S cm⁻¹ at 80 °C.