The invention relates to the field of pesticides, and in particular relates to a preparation method of glufosinate-ammonium. The preparation method of the glufosinate-ammonium comprises the following steps: introducing carbon dioxide into an aqueous solution of hydantoin derivatives until the system pressure is 1 to 4MPa, then heating to 120-180 DEG C for reaction, and after the reaction is finished, carrying out post-treatment to obtain the glufosinate-ammonium. Carbon dioxide is filled into an aqueous solution of the hydantoin derivatives until the system reaches specific pressure, so that more carbon dioxide is dissolved in water, that is, the hydantoin derivatives are in a weak acid environment, hydrolysis of the hydantoin derivatives is promoted by combining specific heating temperature, and meanwhile, generation of impurities is reduced. Inorganic salt is not generated in the whole preparation process, and subsequent desalting operation is not needed; a large amount of waste liquid is not generated, and the prepared glufosinate-ammonium is high in yield and good in purity; in addition, the whole glufosinate-ammonium preparation process is simple and convenient, low in cost and easy to industrialize.