The invention belongs to the technical field of a plant fertilizer, and particularly relates to a special fertilizer for a longan tree and a preparation method thereof. The special fertilizer is prepared by adopting the following raw materials in parts by weight: 35 to 40 parts of excrement of livestock, 25 to 30 parts of starch slag, 20 to 25 parts of mildew feed, 10 to 15 parts of cotton seed meal, 10 to 15 parts of edible mushroom dregs, 5 to 10 parts of humic acid, 155 to 175 parts of urea, 160 to 180 parts of potassium chloride, 50 to 70 parts of potassium magnesium fertilizer, 90 to 100 parts of calcium superphosphate, 10 to 15 parts of lignin, 7 to 9 parts of amino acid chelated copper and boron, 7 to 9 parts of cane sugar complexed manganese and calcium, 3 to 5 parts of diaminoferric xanthohumate and 4 to 6 parts of lignin zinc sulfonate. The special fertilizer is comprehensive in nutrition and stable and lasting in fertilizer efficiency; when the prepared fertilizer is used for fertilizing the longan tree, the nutrition requirement of the longan tree can be met, the growth speed of the longan tree can be increased, the yield and quality of longan can be improved, the characteristics of the soil can be improved, the formation of a soil cluster structure can be promoted, the soil is loosened, the soil structure is improved, the soil fertility is improved, and the environmental protection value and economic benefit are remarkable.