The invention discloses a special organic fertilizer for radish, and a preparation method thereof, wherein the special organic fertilizer for radish is prepared from the following raw materials by weight: 40-60 parts of earthworm manure, 25-35 parts of cow dung, 27-37 parts of oil cake, 1.3-2.3 parts of a fermenting agent, 18-28 parts of straw, and 33-43 parts of water hyacinth. The preparation method comprises: raw material pretreatment, fermentation treatment, and granulating and drying. According to the present invention, with the added earthworm manure, the fertility can be increased, and the quality of the prepared organic fertilizer can be significantly improved; the water hyacinth is added to the raw materials and contains a large amount of trace elements, such that the nutrition components of the organic fertilizer prepared by adding the water hyacinth can be improved, the radish growth requirements can be met, and the problem that the water hyacinth pollutes water resources to cause the high death rate of fishes and shrimps due to hypoxia can be solved; and during the fermenting process, a lot of bacteria in the organic fertilizer are killed through the high temperature, such that the influence of the bacteria on crops is reduced during the application process of the organic fertilizer, the soil environment is purified, and the utilization rate is improved.