Fractionation of lignocellulosic material into its constitutive components is of vital importance for the production of biofuels as well as other value-added chemicals. The conventional acetosolv processes are mainly focused on the production of pulp from woody lignocelluloses. In this study, a mild acetosolv process was developed to fractionate bamboo under atmospheric pressure to obtain cellulosic pulp, water-soluble fraction, and acetic acid lignin. The structural features of the lignins obtained under various conditions were characterized with elemental analysis, sugar analysis, alkaline nitrobenzene oxidation, gel permeation chromatography (GPC), (1)H nuclear magnetic resonance ((1)H NMR), and heteronuclear single-quantum coherence (HSQC) spectroscopy. As compared to milled wood lignin (MWL) of bamboo, acetic acid lignins had low impurities (carbohydrates 2.48-4.56%) mainly due to the cleavage of linkages between lignin and carbohydrates. In addition, acetic acid lignins showed a low proportion of syringyl (S) units. Due to the cleavage of linkages between lignin units, acetic acid lignins had weight-average molecular weights ranging from 4870 to 5210 g/mol, less than half that of MWL (13000 g/mol). In addition, acetic acid lignins showed stronger antioxidant activity mainly due to the significant increase of free phenolic hydroxyls. The lignins obtained with such low impurities, high free phenolic hydroxyls, and medium molecular weights are promising feedstocks to replace petroleum chemicals.