The invention relates to the field of production of chemical fertilizer, in particular to slow-release fertilizer for garden green plants as well as a preparation method thereof. The slow-release fertilizer comprises the following components: garden plant waste, rice bran, a microbial agent, chitin, animal excrement, marine spirillum powder, rotenone, pomace, biogas residues, amino acid, carboxymethylcellulose, ethanol, silicone oil emulsion, potassium persulfate, aziridine, an emulsifier and a proper amount of water. The fertilizer prepared by the method has high persistence, and can effectively reduce loss of fertilizer, increase the utilization rate of the fertilizer, reduce the fertilization times and reduce the labor intensity of farmers and environment burden; meanwhile, the raw materials are selected from environment-friendly waste materials and residues, so that various nutritional elements can be provided for crops; in addition, by the spray-coating method, coated liquid can directly reach to the surface of the fertilizer maximally and is solidified to form a film, so that the distribution uniformity of the film material on the fertilizer granules is greatly improved.