Sorghum stem, an agricultural solid waste discarded in large amounts, was effectively fractionated into its chemical components to achieve value-added utilization. The stem was successively extracted using water at 80 °C and alkali aqueous solutions with increased concentrations (1% NaOH; 60% ethanol containing 1% NaOH, 3% NaOH, 5% NaOH, and 8% NaOH) at 50 °C, which yielded hemicellulose and lignin fractions as well as a cellulose-rich residue. The hemicellulose and lignin fractions were characterized in terms of yield, sugar components, alkaline nitrobenzene, and oxidation analysis. In addition, the molecular weights were determined by gel permeation chromatography and the structures were further identified by Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy. The results indicated that the hemicelluloses yielded from the alkali aqueous solution had a linear xylan structure. The alkali lignin had a typical guaiacyl/syringyl/p-hydroxypheny structure and low amounts of contaminating sugars (less than 2%). A high concentration of alkali aqueous solution led to the release of lignin with a large molecular weight, whereas increasing the alkali concentration resulted in lignin degradation. The residual stem after the successive extractions was rich in cellulose and had a low crystallinity. In sum, mild successive extractions are a promising way to fractionate sorghum stem waste for further conversion.