Abstract: Objective: To prepare acetylated Taraxacum mongolicum polysaccharide from Taraxacum mongolicum Hand.-Mazz, and study its structural characteristics, antioxidant activity and antibacterial activity. Methords: The structure of the polysaccharide extracted by water extraction and then alcohol precipitation method was modified by acetic anhydride method to obtain its acetylated. Infraed spectroscopy, scanning electron microscope and X-ray powder diffraction were used to structural characteristics of the polysaccharide and its acetylated. On the basis, the antioxidant activities were evaluated by 1,1-Dipheny1-2-picrylhydrazyl (DPPH) free radicals, superoxide anion free radicals, hydroxyl free radicals and reducing power scavenging abilities using chemical model method, and the anti-bacteria tests was evaluated through the paper method in vitro. Results: The extraction yield of purified dandelion polysaccharide was 68.75%. Infraed spectroscopy, scanning electron microscope and X-ray powder diffraction of characterization confirmed the successful synthesis of acetylated. The basic structure framework of polysaccharide was not changed after being modified with acetylated. In the concentration range of 0.1～2.0 mg/mL, IC₅₀ values were 5.393±0.941 and 2.153±0.093 mg/mL for pure polysaccharide and acetylated polysaccharides against 1,1-dipheny1-2-picryl-hydrazyl (DPPH) free radicals, respectively, 6.513±0.500 and 2.092±0.825 mg/mL against superoxide anion, respectively, 0.626±0.034 and 0.322±0.010 mg/mL against hydroxyl radical, respectively, and the maximum scanning rates were 0.138±0.019 and 0.239±0.022 reducing power, respectively. As the concentration increased, the antioxidant activity of acetylated polysaccharides increased gradually. In addition, the antibacterial airyctivity of the acetylated polysaccharides was stronger than the unmodified polysaccharides, and the antibaterial activity with the increase of polysaccharide cincentration. Conclusion: The structural stability of purified Taraxacum mongolicum polysaccharide was increased after being modified with acetylated, leading to a significant increase in the antioxidant activity and antibiacterial in vitro.