Liquid hot water was applied to pretreat sunflower stalk to increase its accessibility for enzyme hydrolysis. Pretreatment temperatures, reaction times and pressurized gases were investigated. The results show that the yield of enzyme hydrolysis sugar sun flower stalk is increased after pretreatment. The highest total reducing sugar yield of 83.0% is obtained for the sample pretreated at 200 ℃ and 20 min. Compared with N ₂, CO ₂ pressurized high temperature liquid water treatment at low temperature and short time has catalytic effect. Finally, XRD and SEM analysis are used to investigate the microstructure of the sunflower stalk before and after the pretreatment.