NOVELTY - Method for preparing fertilizer, involves (a) adding sodium hydroxide water solution into polysuccinimide, heating the mixture in water bath at 50-60 degrees C, hydrolyzing the heated mixture for 60-90 minutes, adding calcium chloride (1-2%) and zinc sulfate (1-2%) to the hydrolyzed solution, reacting the obtained solution for 20-30 minutes, adding methanol (250-300%) to obtained solution, reacting the obtained solution for 30-60 minutes at bath temperature, filtering and drying the reacted solution to obtain powdery metallic protease.    USE - The method is useful for preparing fertilizer.    ADVANTAGE - The fertilizer is environmentally-friendly, has improved utilization rate and dissolved phosphorus.    DETAILED DESCRIPTION - Method for preparing fertilizer, involves (a) preparing metallic protease by adding (in parts weight) sodium hydroxide water solution (250-300) of 15% into polysuccinimide (100), heating the mixture in water bath at 50-60 degrees C, hydrolyzing the heated mixture for 60-90 minutes, adding calcium chloride (1-2%) and zinc sulfate (1-2%) to the hydrolyzed solution, maintaining pH value of the obtained solution using hydrochloric acid to 6-8, reacting the obtained solution for 20-30 minutes at bath temperature, stirring the reacted solution, adding methanol (250-300%) to stirred solution, reacting the obtained solution for 30-60 minutes, filtering the reacted solution, and drying the filtered solution at 60 degrees C to obtain powdery metallic protease, (b) adding polyvinyl alcohol (700-1000) of 7% into attapulgite water emulsion of 10%, stirring and reacting the mixture for 30-60 minutes, mixing the reactant with phosphorus dissolving acid in the ratio of 100:0.5-100:1.5, and mixing the obtained mixture with the metallic protease in the ratio of 100:2-100:5 to obtain granule producing agent, where the phosphorus dissolving acid is oxalic acid or nitric acid, and (c) mixing chemical fertilizer with granule producing agent in the ratio of 100:3-100:5, and preparing spherical fertilizer granule.