The invention relates to a water retaining agent for landscaping soil and a preparation method and application of the water retaining agent for landscaping soil. The water retaining agent is preparedfrom, by weight, 60-86 parts of N,N-methylene bisacrylamide-acrylamide-sodium acrylate copolymer, 10-60 parts of sodium-based montmorillonite, 40-100 parts of potassium Polyacrylate, 0.1-1.0 part of across-linking agent, 0.1-1.0 part of an initiator and 1200-2000 parts of deionized water. In the N,N-methylene bisacrylamide-acrylamide-sodium acrylate copolymer, a mass ratio of raw material monomers is (0.5-1):(40-60):(15-25), balance of water absorption rate and water retentivity is realized, high water absorption rate and high water retentivity are achieved, and comprehensive water absorbingperformances of the water retaining agent are improved. In addition, requirements on performances of the water retaining agent in different environments can be met.