A method of preparing a water-soluble and biodegradable antimicrobial agent is provided. The method includes treating a polypeptide compound with sodium hypochlorite for at least 1 min, such that the polypeptide compound has at least one N-halamine group and has good antimicrobial effect. By using the method of the present invention, a chemical synthesis process is not required and an organic solvent is not required either, and thus, an antimicrobial agent having high water solubility, biodegradability, non-toxicity, good sterilization effect, and regeneration can be prepared with reduced harms to the environment.
