The present invention relates 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; adding a surface cross-linking agent to the milled polymer; and elevating the temperature of the polymer including the surface cross-linking agent at a speed of 3°C/min to 15°C/min, and carrying out a surface cross-linking reaction at 100°C to 250°C.