The invention relates to a core-shell ionomer that is used to intensify the crystallization for PET resin and the method for preparing the same. In the beginning, the ionomer employs (methyl) acrylicacid monomer with glass- transition temperature of contained butyl acrylate far lower than room temperature for part polymerization and nucleation, employs monomer with glass- transition temperature of contained methyl methacrylate higher than room temperature and a plurality of unstaratured aliphatic acids containing methacrylic acid for polymerization and nucleation, then forms soft core/hard shell core-shell construction; then, neutralizing with base liquid to establish core-shell ionomer. The invention employs cheap monomer and substance, forms core-shell ionomer through emulsion polymerization, and applies it to the forming and extrusion for PET resin, which toughes and promotes the crystallization for PET, and widens its application in engineering plastic field with lower cost.