The invention belongs to the field of organic chemistry, and relates to preparation of sustained-release organic fertilizers. The organic fertilizer consists of an organic fertilizer body and a water-retention sustained-release carrier, wherein according to the dry weight, the water-retention sustained-release carrier accounts for 2-8 percent of the weight of the organic fertilizer body; the water-retention sustained-release organic fertilizer is a polymer prepared by grafting crylic acid and/or a crylic acid derivative and cellulose-containing tobacco stem slag; and the crylic acid derivative comprises acrylamide or sodium acrylate. The water-retention sustained-release carrier of the organic fertilizer provided by the invention is high in water absorption, long in water retention time, low in cost, environment-friendly and prepared by tobacco waste serving as a raw material; when the water-retention sustained-release carrier is grafted to the crylic acid or the crylic acid derivative thereof to form the polymer, an undesirable water absorption effect is achieved; the organic fertilizer has the advantages of slow release and controlled release and is suitable for fertilization of places which are changeable in climates and are easy to dry.