The invention provides a water-extraction preparation method and an application of an antibacterial active component in bamboo willow. Bamboo willow bark powder is subjected to ultrasonic extraction 1-5 times with distilled water diluted by 5-20 times and for 30-120 min each time, and an extraction solution is obtained; the extraction solution is subjected to alcohol precipitation to remove water-soluble impurities and then separated with macroporous resin, 95% ethanol elution sections are collected, concentrated under reduced pressure and purified with Sephadex LH-20 to remove pigments, and the antibacterial active component in the bamboo willow is obtained. The bamboo willow water extract has better inhibition activity for bacteria such as beta hemolytic streptococcus, staphylococcus aureus, bacilli, escherichia coli, pseudomonas aeruginosa and the like and can serve as a prescribed preparation to be directly used for treating diseases caused by bacterial infection, such as acute tonsillitis, acute pharyngitis, acute laryngitis and acute otitis media. With the adoption of the method, the antibacterial active component is successfully extracted from the bamboo willow and a foundation is laid for application of the bamboo willow in the field of medicine products.