Isolation and utilization of hemicellulose are important processes in the pulp mill-based biorefinery. Considering the potential end use of the hemicellulose, this study investigated the concentration of monomeric and oligomeric sugars in the hydrolysate derived from autohydrolysis of acacia wood and the obtainment of these sugars via ethanol precipitation. The rate of generation of monomers and oligomers increased dramatically with an increase of the temperature from 160℃ to 180℃. The maximum content of oligomers and total sugars was achieved under conditions of 180℃ and 10 min, 170℃ and 85 min, respectively. Interestingly, the maximum yield of hemicellulose via ethanol precipitation was achieved in a much shorter time, compared with the maximum yield of oligomer from the hydrolysate by autohydrolysis. A 6.66 g/L quantity of hemicellulose was obtained by intensifying the conditions of ethanol precipitation. The hemicellulose characteristics were analyzed by Fourier transform infrared, nuclear magnetic resonance, and ultraviolet spectroscopy, and the molecular weight was determined by gel permeation chromatography and thermal analysis.