Abstract: The content changes of polysaccharide in Polygonatum sibiricum wre studied after nine-steam-nine-bask processing. Using 1-Phenyl-3-Methyl-5-Pyrazolone(PMP)as the pre-column derivatization reagent,the monosaccharide composition of Polygonatum polysaccharide was determined by High Performance Liquid Chromatography(HPLC). Subsequently,the in vitro antioxidant activity of Polygonatum sibiricum polysaccharides(PSP)was conducted. It was found that the content of unsteamed PSP was 14.36%. Along with the increase of steaming times,the content of PSP gradually decreased and became stable,remaining at about 4%. Based on the monosaccharide composition analysis,PSP were found to be mainly composed of mannose,glucose,galactose and arabinose,and the percentage ratio of the four monosaccharides without steaming were 52.47%,36.84%,7.32%,3.37% respectively. With the increase of steaming times,the content of mannose decreased,while the contents of galactose and arabinose relatively increased. Interestingly,the content of glucose firstly decreased and then increased. PSP had a certain antioxidant activity,which was significantly increased after nine-steam-nine-bask processing. Although the nine-steam-nine-bask processing caused the loss of some PSP,the isomerization of certain PSP took place under the high humidity and high temperature conditions to result in new monosaccharide composition.