The present invention relates to a superabsorbent polymer that has more improved gel strength and is improved in both water retention capacity (centrifuge retention capacity, CRC) and absorption ability under pressure (AUP) because of an optimized cross-linked structure of a base resin powder inside a cross-linked surface layer, and a preparation method thereof. The superabsorbent polymer includes a base resin powder including a 1st cross-linked polymer of a water-soluble ethylenic unsaturated monomer having acid groups of which at least a part is neutralized, and a cross-linked surface layer that includes a 2nd cross-linked polymer further cross-linked from the 1st cross-linked polymer and is formed on the base resin powder, wherein an inorganic particle is chemically bonded to the 1st cross-linked polymer by the medium of a cross-linking bond, an oxygen-containing bond (—O—), or a nitrogen-containing bond (—NR—, where R is hydrogen or a C1-C3 alkyl or an amide bond).