A method of making water retaining gel, namely, first dissolve sodium hydroxide in water and add acrylic acid for pretreatment; then add corn starch, acrylamide and calcium carbonate in turn, stirring and heating the reaction, then add initiator to carry out the graft polymerization reaction; and then pour the liquid after the reaction into the mold, and dry at a constant temperature can be. Its product components (weight percentage) include: corn starch 4.5-4.7, acrylic acid 21.4-22.5, acrylamide 9-9.5, ammonium persulfate 3.2-4.5, calcium carbonate 4.5- 4.7, sodium hydroxide 9.5-9.9, water remainder. It has the advantages of simple process, non-toxic, biodegradable and wide application.