Bamboo was fractionated by hydrothermal method in ionic liquid-ethanol-H2O system, the dissolution of hydrolysis residue in the system studied, and the structure of separated components analyzed. The results showed that the removal rate of hemicellulose was 82.88% by hydrothermal pretreatment, most of the lignin was dissolved in the solution, but the cellulose was not hydrolyzed. The optimal dissolution conditions were obtained as reaction temperature 170℃, reaction time 4 h, and [AMIM]OAc:C2H6O:H2O=5:5:0.5(j). Under these conditions, the dissolution rate of hydrolysis residue was 43.14%, and the purity of crude cellulose and crude lignin 91.38% and 87.70%, respectively.