Abstract: To study the antimicrobial activities and the difference of active constituents of Forsythia suspense fruit and Forsythia suspense leaves, the diameter inhibition zone of Forsythia suspensa fruit and Forsythia suspensa leaves ethanol extract against bacteria was measured by K-B method, the minimum inhibitory concentration (MIC) was measured by double dilution method. The antibacterial mechanism of Forsythia suspensa fruit and Forsythia suspensa leaves ethanol extract against Staphylococcus aureus was studied by the growth curves and the leakage of cell contents and alkaline phosphatase (AKP). Gas chromatography-mass spectrometry was used to analyze the chemical composition of Forsythia suspensa fruit and Forsythia suspensa leaves ethanol extract. The results showed that ethanol extract from Forsythia suspensa fruit and Forsythia suspensa leaves had antimicrobial effects on Escherichia coli, Staphylococcus aureus and Bacillus subtilis, the diameter inhibition zone of Forsythia suspensa leaves ethanol extract against these bacteria was (12.7±1.2), (15.0±0.8), (14.7±0.7) mm, respectively. The MIC of Forsythia suspensa fruit and Forsythia suspensa leaves ethanol extract against Staphylococcus aureus was 6.25, 3.125 mg/mL, respectively. The experimental results of antimicrobial mechanism showed that the ethanol extract from Forsythia suspensa fruit and Forsythia suspensa leaves inhibited the growth of Staphylococcus aureus, damaged the cell wall and cell membrane, increased the content of macromolecules and AKP in the culture solution, and the antimicrobial activity of Forsythia suspensa leaves ethanol extract was better than Forsythia suspensa fruit ethanol extract. GC-MS analysis showed that, ethanol extract from Forsythia suspensa fruit mainly included alcohols (38.55%), phenols (14.00%), and ketones (11.07%). Forsythia suspensa leaves ethanol extract contained alcohols (34.97%), olefins (12.71%), phenols (12.26%), and alkanes (11.05%). Alcohols, phenols, ketones, aldehydes and alkanes in the Forsythia suspensa fruit and Forsythia suspensa leaves ethanol extract had a significant contribution to their antibacterial activity, and the difference in their antibacterial activity was mainly related to the types and the content of these substances.