As a polymerization solvent for use in the polymerization of monomers for a hydrocarbon-based electrolyte polymer, a mixed solvent containing a first solvent, such as dimethyl acetamide, N-methylpyrrolidone or dimethyl sulfoxide, and an alcohol as a second solvent is used instead of a dimethyl acetamide/toluene mixture or dimethyl acetamide alone according to the related art. By using the mixed solvent, it is possible to obtain a molecular weight of polymer equal to or higher than the molecular weight of polymer obtained from the method according to the related art, even when the reaction time is reduced. Therefore, the disclosed method and polymerization solvent is very useful for a mass production of a hydrocarbon-based electrolyte polymer.