Disclosed is a method for preparing ganirelix acetate. The method includes the following steps: respectively replacing Fmoc-HArg(Et)2-OH and Fmoc-D-HArg(Et)2-OH by employing Fmoc-Lys(Boc)-OH and Fmoc-D-Lys(Boc)-OH or Fmoc-Lys(Alloc)-OH and Fmoc-D-Lys(Alloc)-OH; synthesizing a ganirelix precursor I or ganirelix precursor II-peptide resin in advance; and then respectively performing modifications and treatments on side chain amino groups of Lys and D-Lys in the precursor I or the precursor II-peptide resin to obtain ganirelix acetate. The ganirelix acetate synthesized therefrom is high in purity, has few impurities and a relatively low cost, and is suitable for large-scale production.