Provided is an organic-inorganic polymeric water-retaining fertilizer, which is a co-polymer made by fusion and co-polymerization of an organic water-retaining monomer and an inorganic nutrient under action of a catalyst, a biological enzyme and a modifying agent. The co-polymer is of a three-dimensional mesh-like hydrophilic group structure. In a method for preparing the organic-inorganic polymeric water-retaining fertilizer, after obtaining a neutralized pre-polymer from the organic monomer with the catalyst, an initiator and a cross-linking agent are added; the inorganic nutrient and a metasilicate are added simultaneously, a solution is formed by stirring sufficiently; the biological enzyme is added for catalysis; a co-polymer is obtained after fusion and co-polymerization; and granulation and drying are carried out. Preferably, the catalyst is an inorganic alkaline solution. Preferably, the catalyst is selected from the group consisting of a sodium hydroxide solution, a potassium hydroxide solution, an ammonia water and a calcium hydroxide solution.