Generation of bioenergy, new functional polymers, or chemicals and biomaterials from hemicelluloses are important uses for biomass. In this paper, a novel functional biopolymer with carbon carbon double bond and carboxyl groups was prepared by a homogeneous esterification of xylan-rich hemicelluloses (XH) with maleic anhydride in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) ionic liquid using LiOH as catalyst. The biopolymers with degrees of substitution (DS) between 0.095 and 0.75 were accessible in a completely homogeneous system by changing reaction temperature, reaction time, the dosage of catalyst, and the molar ratio of maleic anhydride to anhydroxylose unit in XH. Results obtained from FT-IR and C-13 NMR spectroscopies confirmed the structure of hemicellulosic derivatives with carbon carbon double bond and carboxyl groups, implying an efficient method to prepare a novel and important functional biopolymer for biomaterials.