A preparation method of water-retaining hydrogel comprises dissolving sodium hydroxide in water and adding acrylic acid for pretreatment; Sequentially adding corn starch, acrylamide and calcium carbonate, stirring and heating for reaction, and adding initiator for graft polymerization; Then the reacted liquid is poured into the mold and dried at constant temperature. Its product components (weight percentage) include: corn starch 4.5-4.7, acrylic acid 2 1.4-22.5, acrylamide 9-9.5, ammonium persulfate 3.2-4.5, calcium carbonate 4.5-4.7, sodium hydroxide 9.5-9.9 and the balance water. It has the advantages of simple process, non-toxicity, biodegradability and wide application.