The degradation rules of glycan in reed ethanol/water cooking process were studied. The degradation rules of polydextrose and polyxylose were obtained in the cooking process by means of ion chromatography determination of glucose and xylose in the reed ethanol pulp content under different holding time. Results showed that cellulose (polydextrose) removal rate obviously was divided into two stages, rapid removal stage at the holding time of 0-90min and slow removal stage at the holding time of 90-210min; hemicellulose (xylan) removal rate was also divided into two stages, slow removal stage at the holding time of 0-60 min and rapid removal stage at the holding time from 60min until the end; meanwhile the change rules of lignin and ash content were achieved.