This study investigated the antimicrobial activity of cinnamaldehyde against drug-resistant Klebsiella pneumoniae using microbroth dilution and plate coating methods, and growth curve determination. The impacts of cinnamaldehyde on cell wall and membrane were evaluated through conductivity, membrane potential and DNA leakage assays. Scanning electron microscopy was employed to observe bacterial morphology changes. The effect of cinnamaldehyde on intracellular protein levels was assessed using SDS-PAGE, and its inhibitory effect on biofilm formation was determined by crystal violet staining. Results indicated that cinnamaldehyde’s minimum inhibitory concentration against drug-resistant K. pneumoniae ranged from 512 to 1024 μg/mL, with a minimum bactericidal concentration between 512 and 2048 μg/mL. Upon cinnamaldehyde exposure, the culture medium’s conductivity and intracellular DNA leakage significantly increased to the maximum within the first hour. Additionally, there was a notable increase in cell membrane potential leading to hyperpolarization. SDS-PAGE electrophoresis revealed a positive correlation between cinnamaldehyde concentration and intracellular protein reduction. Ultrastructural observations demonstrated that cinnamaldehyde damages both the cell wall and membrane, with the degree of damage corresponding to its concentration. Furthermore, biofilm formation by K. pneumoniae was notably inhibited by all sub-inhibitory concentrations of cinnamaldehyde, with the degree of inhibition positively correlating with drug concentration. In summary, cinnamaldehyde exhibited antibacterial effects by impairing the cell membrane and wall, disrupting soluble protein metabolism and cell morphology, and inhibiting biofilm formation.