Sulfonated aromatic polymers (SAPs) feature highly-hydrophilic active functional groups and inherent ionic nano-channels that make them a potential membrane material for desalination. The high content of sulfonic groups favors water transport but causes excessive swelling to impact the separation properties of the membrane. In this work, a novel crosslinking strategy via the linkage of carbon atoms on aryl groups by the Friedel–Crafts reaction was proposed for a newly synthesized sulfonated polystyrene-grafted poly(styrene-ethylene/butylene-styrene) block copolymer (S-SEBS-g-PSt) membrane. Polystyrene was grafted to provide extended side chains of the polymer where the aryl rings were crosslinked using formaldehyde dimethyl acetal (FDA) as the crosslinker to form methylene bridges. Atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS) characterization show that the hydrophilic region of the membrane was mildly shrunken while more continuous after crosslinking. The sulfonic acid groups were retained upon the FDA crosslinking and the high level of ion exchange capacity (IEC) was not affected by the crosslinking. Depending on the crosslinking conditions, the tensile strength of the membrane could be increased by 40–120% and the swelling ratio was reduced by 50–80%. As high as 76.8 kg m⁻² h⁻¹ of water flux and over 99.95% of salt rejection were achieved in treating with 5 wt% hypersaline water at 75 °C. Water permeability reached an unprecedentedly high level of 1 500 000–3 500 000 barrer upon desalinating 3.5–20 wt% salt solutions. The high performance of the membrane makes it promising for potential application in desalination and treating hypersaline wastewater.