A method for preparing sulfonated polystyrene, whereby a styrene monomer having a sulfonyl chloride is prepared by reacting p-styrene sulfonic acid salt with a chloride, a styrene-based polymer having the sulfonyl chloride is synthesized by the polymerization of a p-styrene sulfonyl chloride monomer and optionally at least one monomer of a styrene and an acrylic monomer, the styrene-based polymer having the sulfonyl chloride is treated with an inorganic base aqueous solution to change the sulfonyl chloride into a SO.sub.3M group where M is an alkali metal, and treated with an acidic aqueous solution to prepare a styrene-based polymer having a sulfonic acid group. The styrene-based polymer having the sulfonic acid group prepared according to the process of the present invention has high proton conductivity and good mechanical properties.