In this study， the activity of the cecropin A-melittin hybrid peptide BP21 (Ac-FKLFKKILKVL-NH) in controlling of citrus post-harvest green and blue molds and sour rot and its involved mechanism was studied. The minimum inhibitory concentrations of BP21 against ， ， and were 8， 8， and 4 μmol L， respectively. BP21 could inhibit the growth of mycelia， the scanning electron microscopy results clearly showed that the mycelia treated with BP21 shrank， formed a rough surface， became distorted and collapsed. Fluorescent staining with SYTOX Green (SG) indicated that BP21 could disintegrate membranes. Membrane permeability parameters， including extracellular conductivity， the leakage of potassium ions， and the release of cellular constituents， visibly increased as the BP21 concentration increased. Gross and irreversible damage to the cytoplasm and membranes was observed. There was a positive correlation between hemolytic activity and the concentration of BP21. These results suggest peptide BP21 could be used to control citrus post-harvest diseases.