Due to superior properties, chitosan (CS) has been widely applied in health field. However its acid-solubility and poor antimicrobial activity compromise its safety and applications, respectively. To enhance its water-solubility and antimicrobial activity, gentamicin-grafted-chitosan (GT-CS) is synthesized for the first time, which is then confirmed by various characteristic methods and the graft ratio for gentamicin is about 22.1%. Particularly, it is the first study, disclosing there are degradation and crosslinking involved simultaneously during CS oxidation. Furthermore, GT-CS shows greatly improved water solubility (10.0w%). Compared to CS, the antibacterial activity of GT-CS is improved significantly, approaching the pure gentamicin. It should be noted that the antimicrobic mechanism may not be the target-specific interactions but the nonspecific interactions with the bacterial membrane as shown by scanning electron microscope. To sum up, GT-CS is a superior antimicrobial with good solubility.