NOVELTY - Method for preparing by-product of potassium dihydrogen phosphate, involves heating, stirring and reacting mixture comprising phosphoric acid and carbonamide, mixing and reacting obtained slurry with alkaline potassium salt to obtain a slurry, subjecting the slurry to crystallization and filtration, performing recrystallization of filtrate, filtering and subjecting filtrate to analysis and detection, obtaining by-product of potassium dihydrogen phosphate as filter cake, processing and drying the by-product, and adjusting moisture content of dried product to 3-4%.    USE - The method is useful for preparing by-product of potassium dihydrogen phosphate which is used as special fertilizer for tobacco. The by-product is 2,2-dimethyl-1,3,2,l5-dioxaphosphetan-4-one (all claimed).    ADVANTAGE - The method is environmentally-friendly as it causes no pollution to the environment, and cost-effectively prepares fertilizer. The fertilizer reduces uptake of excess nutrients by soil.    DETAILED DESCRIPTION - Method for preparing by-product of potassium dihydrogen phosphate, involves (a) heating mixture comprising phosphoric acid and carbonamide in a molar ratio of (1.2-1.6):1 at a rate of 5 degrees C/minute until the temperature reaches 80-160 degrees C, and stirring and reacting the mixture for 4-5 hours to obtain slurry, (b) mixing the slurry with 1.1-1.3 mol alkaline potassium salt, and reacting obtained mixture to obtain slurry comprising nitrogen, phosphorus and potassium, (c) adding the slurry comprising nitrogen, phosphorus and potassium to a cooling crystallizer for crystallization, and subjecting obtained product to filtration to obtain filtrate, and (d) circulating the filtrate in cooling crystallizer for recrystallization, performing filtration, subjecting the filtrate to analysis and detection, and stopping recrystallization and filtration when obtained filtrate comprises 15-18% nitrogen, 1.5-4.5% phosphorus pentoxide and 8.1-14.4% potassium oxide, to obtain by-product of potassium dihydrogen phosphate as filter cake, adding microelement composition to the filtrate for adjusting chemical components of the by-product, drying the by-product using dryer at a rate of 3 degrees C/minute until the temperature reaches 45-55 degrees C for 0.5-1 hour, and adjusting moisture content of dried product to 3-4%.