The invention belongs to the field of chemical synthesis, and in particular relates to a novel method for preparing herbicide, namely glufosinate-ammonium. The method comprises the following steps: conducting addition reaction on methyl phosphonic acid ester compounds and DL-2-hydroxy-3-crotonic acid ester compounds to obtain hydroxybutyric acid ester derivatives, and conducting acidification and ammonification reaction on the hydroxybutyric acid ester derivatives to obtain a glufosinate-ammonium compound. The method can avoid using high-toxic cyanide and obviously shortens a reaction route, so that the reaction steps of a process for preparing glufosinate-ammonium are reduced, the operation is simpler and more convenient, and many times of recrystallization is not needed for removing ammonium salt. The cost is reduced, and the method is completely suitable for large-scale production.