The stable Ag NPs protected by chitosan oligosaccharide grafted with polyethylenimine (COS-g-PEI) were prepared, and the antibacterial properties were studied. First, COS-g-PEI was synthesized using ionic liquid 1-butyl-3-methyl imidazolium chloride（[BMIM]Cl）as solvent and catalyst, and 1, 1-carbonyldiimidazole as coupling reagent. Then, Ag NPs@-COS-g-PEI composite particles were prepared by in-situ reduction method using COS-g-PEI as stabilizer, sodium borohydride as reductant. The results of IR and 1H-NMR showed that COS-g-PEI was prepared successfully. And the results of UV-Vis and TEM showed that the stable Ag NPs@-COS-g-PEI composite particles were prepared. The antimicrobial properties of COS-g-PEI and Ag NPs@-COS-g-PEI for staphylococcus aureu and escherichia coli were evaluated by Kirby-Baner test and oxford cup method. Ag NPs@-COS-g-PEI showed the larger diameter of inhibiting zone than COS-g-PEI against staphylococcus aureu and escherichia coli no matter by Kirby-Baner test or oxford cup method. It is suggested that the antibacterial properties of COS and Ag NPs are combined together to give higher antibacterial ability.