A series of raw, iodine (bromide) or/and sulfuric acid-modified chitosan sorbents were synthesized and comprehensively characterized by N-2 isotherm adsorption/desorption method, TGA, FTIR, XRD, and XPS et al. Adsorption experiments of vapor-phase elemental mercury (Hg-0) were studied using the sorbents in a laboratory-scale fixed-bed reactor. The results revealed that porosities and specific surface areas of the sorbents decreased after modification. The sorbents operated stably at flue-gas temperature below 140 degrees C. The chemical reactions of iodine and sulfate ion with the amide of chitosan occurred, and the I-2 was found in the sorbents due to the presence of H2SO4. Fixed-bed adsorber tests showed that compared to raw chitosan, the bromide or iodine-modified chitosan could promote the efficiency of Hg-0 capture more or less. Mercury removal efficiency could be significantly promoted when an appropriate content of H2SO4 was added, and the iodine and H2SO4 modified sorbents almost had a mercury removal efficiency of 100% for 3 h. The presence of moisture can increase the sorbent's capacity for mercury uptake due to the existence of active sites, such as sulfonate and amino group. The mercury breakthrough of modified chitosan sorbents decreased with increasing temperature. A reaction scheme that could explain the experimental results was presumed based on the characterizations and adsorption study.