Abstract: To understand the enrichment characteristics of trace As in fly ash, the adsorption mechanism of AsO, a typical arsenic oxide, in SiO₂, the main component of fly ash, was investigated by using density functional theory; energy calculation, AIM theory, Mulliken charge analysis and Localized Orbital Locator(LOL) color map were performed on the optimized adsorption configuration, in order to analyze the interaction between AsO and SiO₂. The results show that the adsorption energy of AsO on the defect sites of amorphous SiO₂ is higher than 50 kJ/mol, typical for the configuration of chemical adsorption. The bonds of As-Si, Si-O and As-O formed at the active defect sites of the amorphous SiO₂ have high strength, belonging to the covalent bond; that is, the interaction between SiO₂ and AsO is covalent.