The complicated structures of bagasse hinder the bioconversion processes for the production of bioenergy and biomaterials. In this study, an integrated process of hot-water extraction followed by high-boiling-solvent cooking (HBS, i.e., 1,4-butanediol) was demonstrated to fractionate bagasse into hemicellulose, lignin, and cellulose. The hot-water extraction resulted in the removal of hemicellulose, which facilitated the HBS cooking for the open fiber structure. As a result, 57-70% of lignin was isolated from the cooking spent liquor. Gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), P-31 and H-1 nuclear magnetic resonance (NMR), and thermogravimetry (TG) were employed for characterization of the organosolv lignin. Results showed that the organosolv lignin exhibited a chemical structure similar to enzymatic hydrolysis/mild acidolysis lignin (EMAL) but formed new phenolic OH groups (3-6-fold of EMAL). The high molecular weight and thermal stability would contribute to the potential application of lignin into value-added products. The proposed processes provide an efficient approach for fractionating the three main components of bagasse, especially lignin.