The arsenic (As) vitrification in copper smelting slag derived from the FeO-SiO₂-Fe₂O₃-B₂O₃-CaO- Al₂O₃-(MgO) system was studied. The vitrification mechanism of As was systematically investigated by XRD, SEM, FTIR and XPS analysis. It is determined that As exhibits favorable compatibility with the iron-silicate slag. FTIR and XPS analysis indicates that As can partially substitute for silicon to form the Si—O—As structure, thereby enhancing the degree of polymerization of the melt. The [BO₃]^3- unit tends to decrease with the increase of the As content, while the opposite behavior is observed with the addition of MgO. Additionally, MgO increases the non-bridging oxygen (NBO) number of the melt. TCLP, SPLP and pH dependence tests show that the leaching toxicity of As increases with the increase of the As content but decreases with the addition of MgO. DSC analysis demonstrates that the incorporation of both As and MgO can improve the thermal stability of the As-containing glass.