The invention discloses a slow-release fertilizer special for cotton and a preparation method thereof. The slow-release fertilizer special for the cotton is prepared from the following raw materials: by weight, 12-15 parts of urea, 12-15 parts of ammonium chloride, 6-9 parts of potassium chloride, 5-8 parts of monoammonium phosphate, 1-2 parts of copper sulfate, 1-2 parts of sodium selenite, 1-2 parts of ferrous sulfate, 2-3 parts of quicklime, 5-6 parts of meerschaum, 2-3 parts of calcium carbonate, 1-2 parts of melia root bark powder, 1-2 parts of drug residue ash, 0.01-0.02 part of compound sodium nitrophenolate, 2-3 parts of brown sugar and 1-2 parts of waste engine oil. In the slow-release fertilizer special for the cotton, through a bonding effect of the brown sugar, the copper sulfate, the sodium selenite and the ferrous sulfate are mixed, so that when the fertilizer releases, balance fertilizer efficiency can be guaranteed. The waste engine oil can coats all the fertilizer components so as to play a slow release effect. The quicklime can improve heavy metal pollution of soil, can combine heavy metals to deactivate the heavy metals, and can improve growth environment and conditions of the cotton. Because of reasonable allocation and slow release of nutrient elements such as nitrogen, phosphorus, potassium and various trace elements, the nutrient elements such as the nitrogen, the phosphorus, the potassium and the various trace elements are beneficial for growth of the cotton. The cotton yield can be increased by 20% to 25%, and diseases and insect pests during cotton growing periods can be decreased.