NOVELTY - Preparing water-soluble fertilizer, comprises (i) adding polyaspartic acid or its salt to normal water-soluble fertilizer, where the molecular weight of polyaspartic acid or its salt is 2500-10000 Daltons; (ii) uniformly mixing aqueous solution of polyaspartic acid or its salts with common liquid water-soluble fertilizer, and stirring, to obtain a liquid water-soluble fertilizer; and (iii) powdering aqueous solution of polyaspartic acid or its salts by spray drying method, and uniformly mixing with common solid water-soluble fertilizer, and obtaining a solid water-soluble fertilizer.    USE - The method is useful for preparing water-soluble fertilizer.    ADVANTAGE - The method has synergistic effect; can improve the utilization ratio of water-soluble fertilizer, reduces the usage amount of fertilizer, is environmental-friendly, and saves the cost of agricultural production.    DETAILED DESCRIPTION - Preparing water-soluble fertilizer, comprises (i) adding polyaspartic acid or its salt to normal water-soluble fertilizer prepared according to national standard or enterprise standard of common water-soluble fertilizer, to make the mass percentage content of the polyaspartic acid or its salt in the water-soluble fertilizer to reach up to 0.05-40%, where the molecular weight of polyaspartic acid or its salt is 2500-10000 Daltons, and salts of polyaspartic acid comprises sodium salt, potassium salt, ammonium salt, zinc salt, magnesium salt, and/or calcium salt; (ii) uniformly mixing the aqueous solution of polyaspartic acid or its salts with a common liquid water-soluble fertilizer, and stirring, to obtain a liquid water-soluble fertilizer; and (iii) powdering aqueous solution of polyaspartic acid or its salts by spray drying method, and uniformly mixing with common solid water-soluble fertilizer to obtain a solid water-soluble fertilizer.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of the device for preparing solid water-soluble fertilizer.    Blower (1)    Heater (2)    Feeding pump (3)    Spray dryer (5)    Air filter (9)