The invention provides a specialized organic / inorganic composite biological medicine fertilizer for watermelon, and a preparation method thereof. The weight parts of composition raw materials are as follows: 50-70 parts of sludge produced by sewage treatment, 20-30 parts of tree ash, 30-40 parts of watermelon vines, 20-30 parts of cow manure, 15-20 parts of bagasse, 5-7 parts of fishbone dust, 8-10 parts of oil cakes, 10-15 parts of the tree ash, 10-15 parts of phosphorus slag, 8-10 parts of poultry feather, 8-10 parts of monoammonium phosphate, 10-12 parts of triple superphosphate, 12-15 parts of potassium chloride, 2-4 parts of carboxymethyl chitosan, 20-30 parts of modified bentonite, 10-12 parts of decomposed coal, 2-4 parts of seven leaf clover, 9-11 parts of angled bittersweet, 8-10 parts of capillary artemisia, 5-7 parts of fructus cnidii, 6-8 parts of fructus quisqualis, 3-5 parts of wormwood, 3-5 parts of litsea cubeba, 8-10 parts of daturae flower powder, 3-5 parts of paraffin, 1-1.25 parts of zinc sulfate, 2-3 parts of ferrous sulfate, 0.8-1.2 parts of borax, 10-12 parts of potassium fulvic acid, 3-4 parts of chili oil and 0.2-0.5 part of EM strains. A green organic fertilizer is obtained by mixing a plurality of organic wastes and fermenting the mixture, thereby reducing environment pollutions. The specialized organic / inorganic composite biological medicine fertilizer for the watermelon has the advantages of comprehensive nutrition and enhanced sterilizing effects by mixing the fermented organic materials and inorganic fertilizer and adding medicine juice of the Chinese herbal medicine having insectidical functions. With the application of the fertilizer, watermelon quality can be effectively increased; and the watermelon has green leaves, bright flower, prolonged flowering period and increased output.