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Gaseous arsenic emitted from coal combustion flue gas (CCFG) causes not only severe contamination of the environment but also the failure of selective catalytic reduction (SCR) catalysts in power plants. Development of inexpensive and effective adsorbents or techniques for the removal of arsenic from high-temperature CCFG is crucial. In this study, halloysite nanotubes (HNTs) at low price were modified with CuCl 2 (CuCl 2 -HNTs) through ultrasound assistance and applied for capturing As 2 O 3 (g) in simulated flue gas (SFG). Experiments on arsenic adsorption performance, adsorption mechanism, and adsorption energy based on density functional theory were performed. Modification with CuCl 2 clearly enhanced the arsenic uptake capacity (approximately 12.3 mg/g) at 600 °C for SFG. The adsorbent exhibited favorable tolerance to high concentrations of NO x and SO x . The As 2 O 3 (III) was oxidized and transformed into As 2 O 5 (V) on the CuCl 2 -HNTs. The Al-O bridge had the highest adsorption energy for the O end of the As-O group (-2.986 eV), and the combination formed between arsenic-containing groups and aluminum was stable. In addition, the captured arsenic could be stabilized in the sorbent at high temperature, making it possible to use the sorbent before the SCR system. This demonstrates that CuCl 2 -HNTs is a promising sorbent for arsenic oxidation and removal from CCFG.