The invention discloses a slow release composite fertilizer and a preparation method thereof. The slow release composite fertilizer is prepared from, by weight, 20-40 parts of urea formaldehyde, 5-13 parts of potassium chloride, 5-13 parts of potassium nitrate, 7-15 parts of ammonium sulfate, 8-16 parts of a medium and trace element chelating solution, 20-30 parts of water, 6-14 parts of monoammonium phosphate and 1-5 parts of a coating material. The preparation method comprises the following steps: mixing above raw materials, and granulating the obtained mixture to obtain the slow release composite fertilizer. The slow release function of nitrogen and phosphorus elements is realized after coating treatment of an organic nitrogen fertilizer and a phosphate fertilizer, and the medium and trace element chelating solution prepared through combining an aqueous ammonium polyphosphate solution obtained by mixing ammonium polyphosphate and water with medium and trace elements is adopted to greatly improve the fertilizer effect of the slow release composite fertilizer, so nutrition components required in the growth process of crops are completely met, and growth of the crops is guaranteed; and raw materials adopted in the invention are nontoxic and harmless, so the slow release composite fertilizer is a green and environmentally-friendly slow release composite fertilizer.