The invention discloses a method for preparing bleomycin. The method comprises the steps that bleomycin fermentation liquor filtrate is subjected to crude purification through macroreticular resin, synchronous fine purification and decoppering are carried out through a C18 reversed phase chromatography column, desalting is carried out through the macroreticular resin, and bleomycin sulfate or hydrochloride with the purity larger than 90% is obtained. According to the method, a C18 reversed phase chromatography refining process integrating fine purification and decoppering is adopted, the quality of prepared bleomycin meets the relevant standard of United States Pharmacopeia, the total yield is larger than 50%, little organic solvent is used in the whole preparation process, safety and environmental friendliness are achieved, the process is simple and practicable, and the method is suitable for large-scale industrial production.