The invention discloses an organic/inorganic slow-release fertilizer for flowers and a preparation method of the slow-release fertilizer. The organic/inorganic slow-release fertilizer is prepared from the following raw materials in parts by weight: 45 to 55 parts of horse manure, 20 to 30 parts of wheat bran, 15 to 25 parts of brewer's grains, 12 to 18 parts of flour beetle frass sand, 10 to 15 parts of residual tea, 5 to 10 parts of edible fungi waste, 18 to 26 parts of sorgo straw, 10 to 15 parts of mung bean shells, 5 to 10 parts of sugar beet meal, 1 to 3 parts of spawn powder, 16 to 22 parts of yellow phosphorus furnace slag, 20 to 30 parts of obsidian, 17 to 24 parts of hectorite, 10 to 15 of parts zeolite 4A, 32 to 44 parts of ammonium nitrate, 22 to 28 parts of ammonium hydrogen phosphate ammonium dihydrogen phosphate, 8 to 14 parts of potassium carbonate, 2 to 3 parts of diaminoferric xanthohumate, 3 to 6 parts of sugar calcium, 4 to 8 parts of sodium metasilicate, 15 to 20 parts of polyvinyl alcohol, 5 to 10 parts of carboxymethyl cellulose, 9 to 16 parts of hydroxypropyl xanthan gum, 2 to 4 parts of algal polysaccharides, and 3 to 5 parts of polyglutamic acid. The fertilizer provided by the invention is high in nutrient amount and high and long in fertilizer efficiency, can improve soil, enables soil to form granular structures, and harmonizes water and gas in soil to enable flowers to grow up well and quickly, thereby improving the quality and the yield of flowers and lowering the fertilization cost; important economic benefits are brought.