The present invention relates to a preparation method of a superabsorbent polymer, and specifically to a method of preparing a superabsorbent polymer including the steps of: preparing a hydrous gel phase polymer by thermal polymerizing or photo-polymerizing a monomer composition including a water-soluble ethylene-based unsaturated monomer and a polymerization initiator; drying the hydrous gel phase polymer; milling the dried polymer; classifying the milled hydrous gel phase polymer into two or more grades by particle size; adding a surface cross-linking agent to each hydrous gel phase polymer classified into two or more grades; and carrying out a surface cross-linking reaction of the hydrous gel phase polymer to which the surface cross-linking agent is added.