NOVELTY - A raw material is subjected to ice-water bath cooling and stirring, and neutralized with sodium hydroxide solution. A dehydrated MS culture medium is added to reactor, and acrylamide solution, sodium fulvate, sodium o-nitrophenol and crosslinker chosen from N,N-methylenebisacrylamide and potassium persulfate are further added to reactor and stirred. Nitrogen is supplied to reactor and slowly heated at 65-85 degrees C for 1 hour. The reaction mixture is cooled to room temperature, and product is removed, dried, crushed and sieved, to obtain composite resin water retention agent.    USE - Preparation of composite resin water-retention agent used for regulating growth of plants in acid/alkali environment.    ADVANTAGE - The composite resin water-retention agent having excellent nutrient absorption efficiency is prepared by simple method, without environmental pollution.    DETAILED DESCRIPTION - A raw material is subjected to ice-water bath cooling and stirring, and neutralized with sodium hydroxide solution, to remove acrylic acid in polymerization inhibitor, to obtain acrylic acid or its sodium salt solution. A dehydrated MS culture medium is added to reactor, and acrylamide solution, sodium fulvate, sodium o-nitrophenol and crosslinker chosen from N,N-methylenebisacrylamide and potassium persulfate are further added to reactor and stirred at room temperature for 1-1.5 hours. The mass ratio of acrylic acid, acrylamide, dehydrated MS culture medium, sodium hydroxide, sodium o-nitrophenol, sodium fulvate, N,N-methylenebisacrylamide and potassium persulfate is 1:0-0.5:0.2-3.2:0.15-0.62:0.0002-0.0599:0.0002-0.0599:0.0002-0.0045:0.0016-0.0089. Nitrogen is supplied to reactor and slowly heated at 65-85 degrees C for 1 hour in water bath, and further heated in boiling water bath for 1.5 hours. The supply of nitrogen is stopped. The reaction mixture is cooled to room temperature, and product is removed, dried, crushed and sieved, to obtain composite resin water retention agent.