D-xylose, a hemicellulose model compound, was oxidized by chlorine dioxide under simulated bleaching conditions, and the mechanism of this reaction was investigated. The final reaction product, chloroacetic acid, was detected by gas chromatography-mass spectrometer (GC-MS). To study the generating mechanism of chloroacetic acid by D-xylose during chlorine dioxide bleaching, three reaction pathways were designed. The results showed that the biggest heat of reaction, -234.33 kJ/mol, and the minimum reaction activation energy, 44.44 kJ/mol, appeared for one of the candidate pathways (no. 2). That pathway was thermodynamically more favored. Xylitol was generated by D-xylose degradation, and then chloroacetic acid was generated by a series of oxidation, fracture, and substitution reactions on xylitol.