Bamboo residue was treated with an active oxygen (O2) and solid alkali (MgO) (CAOSA) process, which was developed recently by the authors owing to its environmentally friendly and high-efficiency characteristics. During the cooking process, 93.0% of lignin and 62.1% of hemicellulose were removed from the raw material, which resulted in a cellulose-rich pulp. This indicated that this cooking process is efficient to fractionate the bamboo residue into cellulose, hemicellulose, and lignin as a pretreatment for biomass conversion. The structural features of the hemicellulose from the pretreated bamboo residue were analyzed for comparison with that of the raw materials. The molecular structure of the hemicellulose fractions obtained from both the raw bamboo and pulp consisted of a (1→4)-β-D-Xylp backbone substituted with α-L-Araf and 4-O-methyl-α-D-glucuronic acid. The hemicellulose with more side chains tended to be more easily removed from the bamboo cell wall during the CAOSA process. Furthermore, the fractions of hemicellulose exhibited much lower thermal stabilities after the cooking process than the raw material.