A series of canthin-6-one analogues were designed and synthesized in order to study their antibacterial activity and structure-activity relationships. Compound 22 showed a broad spectrum of antibacterial activity and exhibited better bactericidal effect (8-fold superiority against Staphylococcus aureus and 2-fold superiority against Ralstonia solanacearum) than fosfomycin sodium and propineb with a minimum inhibitory concentration value of 2 μg/mL. Moreover, it showed low cytotoxicity, stimulation on germination rates and good "drug-like" properties. Membrane-active mechanism was further studied by fluorescent microscopy, scanning electron microscopy, cytoplasmic β-galactosidase leakage assay and evaluation of the molecular docking. The results showed that 22 may exert its bactericidal effect by damaging bacterial cell membranes and influencing the membrane formation, both of which could lead to cell death. The in vivo antibacterial assay with a protective efficacy of 68% demonstrated the potential of C ring-truncated canthin-6-one 22 as a new bactericide.