A method for preparing a polymer is provided. The method for preparing a polymer includes subjecting at least one monomer having a structure represented by Formula (I) to a reaction in the presence of sulfonic acid, diphenyl amine, and oxygen-containing phosphide, obtaining a sulfonium salt polymer wherein x is 0, 1, or 2; R1 is C1-6 alkyl group; and R2 is independently hydrogen, or C1-6 alkyl group. In particular, the molar ratio of the diphenyl amine to the oxygen-containing phosphide is from 4:1 to 1:1.