CaSiO₃ is highly resistant to sintering and can trap arsenic at high temperatures in the boiler furnace. However, the trapping capacity of CaSiO₃ for arsenic does not meet the requirements of practical applications, and it is easy to react with acidic gases, which significantly affects the adsorptive property of arsenic. In this paper, the effect of Al modification on the As₂O₃ adsorption behaviour on the CaSiO₃(001) surface was systematically investigated using a density functional theory. By comparing the magnitude of adsorption energy of different sites, the active site of As₂O₃ adsorbed on the surface of CaSiO₃(001) was determined to be Ca, and the adsorption activity of As₂O₃ by the silicon oxygen chain composed of [SiO₄] tetrahedron is deficient. The Si atoms in the [SiO₄] tetrahedral structure are directly replaced by Al atoms, the difference in bond length and bond energy between Al–O bond and Si–O bond is used to promote the redistribution of surface charge and the increase of local structural bond angle of CaSiO₃(001), leading to the exposure of new active sites (Si-top and Al-top sites) on the silicon oxygen chain. The new active site can realize the chemical adsorption of As₂O₃, the higher adsorption energy of the Al-top site is attributed to the stronger s-p orbital hybridization between Al and O atoms after doping, which is more conducive to the charge transfer between As₂O₃ and the adsorbent surface. In this work, influence of SO₂ and HCl gases on the adsorption of As₂O₃ by modified silicon oxygen chains was also discussed. The results show that SO₂ and HCl in the flue gas may occupy the Al-top site on the silicon oxygen chain through chemical adsorption, and reduce the activity of this site, thereby affecting the adsorption of As₂O₃. However, the exposed Si-top sites owing to Al doping show good acidic gas resistance, which in turn help the surface of Al–CaSiO₃(001) can also maintain stable adsorption of As₂O₃ in SO₂ and HCl atmosphere.