Silver nanoparticles have attracted much attention in the medical field in recent years due to their excellent antibacterial properties. In this paper, size controllable silver nanoparticles were synthesized using Hibiscus sabdariffa extract (HSE) as a reductant in 10 min. The silver nanoparticles prepared using HSE as reductant have an average diameter of 15 nm, which is smaller than that of using citric acid as a reductant. Characterization results show that polyphenol compounds in HSE acts not only as reductant but also as dispersant and stabilizer. Moreover, the size of silver nanoparticles is mainly affected by pH value, reaction temperature, and kind of pH regulators. Finally, the silver nanoparticles as antibacterial component was added into cataplasm. The results showed that they have almost no cytotoxicity on 3T3 cells and excellent antimicrobial activities against Gram-positive and Gram-negative bacterium. The above research opens a feasible way for the application of silver nanoparticles in the biomedical field.