Iron-based sorbent addition is a promising method for arsenic removal from coal-fired flue gas, but the adsorption process and surface active site that are responsible for arsenic adsorption remain unclear. In this work, quantum chemistry methods based on the density functional theory are carried out to explore the mechanism of As2O3 adsorption on the Fe2O3 (001) surface. The results indicate that O-top and O-hollow sites served as the active sites for As2O3 adsorption on the alpha-Fe2O3 (001) surface; among these, the activity of the O-top is higher. The critical step of As2O3 adsorption lies in the bond breaking of the As-O bond in As2O3 molecule, which is confirmed by comparing binding energies of different adsorption sites. The previous experimental studies have proven that O-2 and SO2 have a significant impact on arsenic adsorption, and herein, deep insights into arsenic adsorption in the presence of the above gas components are also included. Under the influence of oxygen, the conversion of the original Fe-top site into the O site results in chemisorption between arsenic and the alpha-Fe2O3 (001) surface, which is the primary cause for the promoting action of O-2. In the presence of SO2, the adsorption activity of the original Fe-top site is enhanced by the newly formed S-ads-top site. In addition, the As adsorption capacity of the original O-top site had also been promoted because of the SO2 adsorption.