The O-2,4-dichlorophenoxyacetic-oligochitosan (Dcpo-O-COS) was synthesized by chitosan oligosaccharide (COS) and 2,4-dichlorophenoxyacetic acid. The steps were as followed: amino protecting of chitosan oligosaccharide, carboxyl acyl chlorination of 2,4-dichlorophenoxyacetic acid, hydroxyl of COS reacted with acyl chlorination 2,4-dichlorophenoxyacetic acid, amino deprotection. Fourier transform infrared (FTIR), UV-Vis absorption and ¹H nuclear magnetic resonance (NMR) were used to characterize the structure of the final product. The results exhibited that Dcpo-O-COS was successfully synthesized. Moreover, both stability, tested by X-ray diffraction (XRD) and Thermogravimetric Analysis (TGA), and antibacterial activity, toresistance to Escherichia coli and Staphylococcus aureus, of Dcpo-O-COS were higher than COS.