Currently, the strategies for prevention and treatment of bacterial diseases in fish are dominated by the use of drugs such as antibiotics. However, the overuse of antibiotics will continue to cause the development of resistant strains. This leads to ineffective treatment with antibiotics during disease breakouts in aquatic animals. Therefore, research into the development of antibiotics that are low in toxicity and not susceptible to resistance has become an urgent task. Increasing studies on Chinese herbal medicines and their extracts have been performed for the prevention and treatment of bacterial diseases in aquaculture. Fraxetin is a natural simple coumarin compound with good antibacterial effects and inhibitory effects against . To provide evidence for the prevention and control of aquaculture diseases, we studied the antibacterial effect of fraxetin on and on the virulence of pathogenic bacteria. Additionally, the minimum inhibitory concentration (MIC), growth curve, biofilm, lipase activity, motility, protease activity, and hemolysis were evaluated. We determined the MIC using the micro-broth method. The growth curve was determined by OD₆₀₀; biofilm formation was detected by crystal violet staining; bacterial liquid was spotted onto solid medium to determine lipase activity and motility; and the co-culture supernatant was collected to determine protease activity and hemolytic activity. The results showed that fraxetin had significantly inhibited was clear and transparent after 24 h, when the concentration of fraxetin was 128 μg/mL and 256 μg/mL. Therefore, the MIC was 128 μg/mL. The inhibitory effect of fraxetin on was concentration-dependent. Fraxetin significantly inhibited the growth of <0.05). At 8 μg/mL, fraxetin did not significantly inhibit the growth of >0.05). The logarithmic growth phase was apparently delayed (<0.05), and the concentration of bacterial solution in the plateau was significantly lower than that in the control group, when the concentration of fraxetin was more than 32 μg/mL. When the fraxetin concentration exceeded 16 μg/mL, <0.05). At 8 μg/mL, fraxetin reduced the lipase activity of <0.05). It had a non-significant effect on the motility and hemolytic activity of >0.05); however, fraxetin at these concentrations could decrease the protease activity of <0.05). Fraxetin presented antibacterial activity against A. hydrophila virulence was limited at non-bacterial concentrations. The prevention and control of by low concentrations of fraxetin requires further study; as the drug affects the pathogenic microorganisms as well as the host, considering the complexity of the effects of Chinese herbal medicines in the body.