The crosslinked sulfonated poly (bis-A)-sulfone (SPSF) proton exchange membranes were prepared by immersing the SPSF membrane into the presence of phosphorous pentoxide-methanesulfonic acid (PPMA) in the ratio of 1:10 by weight. The occurring of crosslinking reaction was proved and the membrane properties before and after crosslinking were evaluated. The results showed that the crosslinking treatment has reduced the water uptake, depressed the swelling, and enhanced the dimensional stability of SPSF membranes with just only slight sacrifice in proton conductivity. The ordered arrangement of molecular chain internal SPSF membrane was reduced and the efficiency of crosslinking reaction showed a random property along the depth direction of the membrane in the process of crosslinking.