The invention relates to the field of chemical technique, especially to a production method for preparing monocalcium phosphate and a by-product a special fertilizer for paddy rice. By changing coefficients such as temperature and the like of wet-process phosphoric acid preparation of monocalcium phosphate, purity of the product monocalcium phosphate is raised, and a compound fertilizer containing N, P and K combined forms can simultaneously be produced. By the method, purity of monocalcium phosphate can be raised, and the produced special fertilizer for paddy rice has good absorptivity. Meanwhile, formaldehyde is added into a filtrate to prepare a controlled-release compound fertilizer. The produced special fertilizer for paddy rice can meet nutritional requirements of each period of growth cycle. By the use of the compound fertilizer, yield of paddy rice is raised greatly. The special fertilizer is an efficient compound fertilizer.