NOVELTY - A water-soluble potassium ammonium phosphate fertilizer is prepared by wet-processing potassium sulfate and phosphoric acid; controlling temperature, solid-liquid separating to obtain fluorophosphate filtrate and potassium fluorosilicate cake; adding ammonia and phosphoric acid, subjecting to neutralization reaction, adjusting pH, and monitoring the reaction; solid-liquid separating; adding sulfuric acid or phosphoric acid; adding solid potassium sulfate material, reacting, gradually cooling, and completely precipitating crystals; solid-liquid separating to obtain third filtrate and crystal.    USE - Method for preparation of water-soluble potassium ammonium phosphate fertilizer (claimed).    ADVANTAGE - The fertilizer has high purity, can produce excess nitrogen potassium sulfate fertilizer, and the byproduct in the production process can be directly used as citrate soluble phosphate fertilizer. The method is simple and environment-friendly, has no waste residue discharge and has good social and economic benefits.    DETAILED DESCRIPTION - A water-soluble potassium ammonium phosphate fertilizer is prepared by wet-processing potassium sulfate and phosphoric acid with molar ratio of 1-1.6:6; controlling temperature at 30-70 degrees c for 0.5-2 hours, solid-liquid separating to obtain fluorophosphate filtrate and potassium fluorosilicate cake; adding ammonia and phosphoric acid, subjecting to neutralization reaction, adjusting pH value by 4.6-6.5, and monitoring the reaction to obtain slurry; solid-liquid separating to obtain second filtrate comprising monoammonium phosphate and diammonium phosphate, and water-insoluble cake comprising iron, aluminum and magnesium phosphate; adding water-insoluble cake with sulfuric acid or phosphoric acid to adjust pH by 3-4.6 to obtain clear liquid; adding with solid potassium sulfate material with molar ratio of 1-2:1, reacting at 70-100 degrees C for 0.5-2 hours, gradually cooling at 10-30 degrees C for 0.5-1 hour, and completely precipitating crystals; solid-liquid separating to obtain third filtrate comprising ammonium sulfate, ammonium dihydrogen phosphate, potassium dihydrogen phosphate and potassium sulfate mixed solution, and a crystal which is water-soluble dried ammonium dihydrogen phosphate fertilizer.