The invention relates to the technical field of chemical industry, and discloses a method for preparing ammonium dihydrogen phosphate and co-producing an enhancement fertilizer special for onions. The method comprises the following steps: analyzing the proportion of nutritive elements required in a growth process of the onions by detecting the quantities of various nutritive elements absorbed from soil in the growth process of the onions, then reacting phosphoric acid with carbamide to generate an intermediate, then introducing ammonia gas into the intermediate to produce ammonium dihydrogen phosphate, separating part of ammonium dihydrogen phosphate by crystallization for adjusting the proportion of the nutritive elements in a reaction liquid to meet the requirements of the growth of the onions, and adding other nutritive elements required by the onions to prepare the enhancement fertilizer special for the onions. According to the production method disclosed by the invention, raw materials can be fully used, produced ammonium dihydrogen phosphate is relatively good in quality, and the prepared enhancement fertilizer special for the onions is relatively low in cost.