The invention discloses a preparation method of a bioorganic fertilizer. The preparation method comprises the following steps: respectively crushing corn stalks, wheat straws and sorghum straws, uniformly mixing the obtained crushed materials, and adding water to soak the obtained mixture to obtain a mixed straw material; uniformly mixing the mixed straw material, a mixed bacterial agent, pig manure, cow dung, sheep manure, distillers'grains, urea, soybean meal, corn flour, brown sugar and water to obtain a mixture; adding the mixture into a reaction kettle, covering the reaction kettle with akettle lid, carrying out heating fermentation, turning the obtained heap, and taking out the obtained decomposed mixture in order to obtain a fermented material; and drying the fermented material, granulating the dried fermented material, and bagging the granulated fermented material to obtain the bioorganic fertilizer. The preparation method has the advantages of short fermentation period, smalloccupied land and low labor cost, and the prepared bioorganic fertilizer has the advantages of good fertilizer effect and high quality.