Abstract: The metallurgical properties of the CaO–SiO₂–Al₂O₃–4.6wt%MgO–Fe₂O₃ slag system, formed by the co-treatment process of spent automotive catalyst (SAC) and copper-bearing electroplating sludge (CBES), were studied systematically in this paper. The slag structure, melting temperature, and viscous characteristics were investigated by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, FactSage calculation, and viscosity measurements. Experimental results show that the increase of Fe₂O₃ content (3.8wt%–16.6wt%), the mass ratio of CaO/SiO₂ (m(CaO)/m(SiO₂), 0.5–1.3), and the mass ratio of SiO₂/Al₂O₃ (m(SiO₂)/m(Al₂O₃), 1.0–5.0) can promote the depolymerization of silicate network, and the presence of a large amount of Fe₂O₃ in form of tetrahedral and octahedral units ensures the charge compensation of Al³⁺ ions and makes Al₂O₃ only behave as an acid oxide. Thermodynamic calculation and viscosity measurements show that with the increase of Fe₂O₃ content, m(CaO)/m(SiO₂), and m(SiO₂)/m(Al₂O₃), the depolymerization of silicate network structure and low-melting-point phase transformation first occur within the slag, leading to the decrease in melting point and viscosity of the slag, while further increase causes the formation of high-melting-point phase and a resultant re-increase in viscosity and melting point. Based on experimental analysis, the preferred slag composition with low polymerization degree, viscosity, and melting point is as follows: Fe₂O₃ content of 10.2wt%–13.4wt%, m(CaO)/m(SiO₂) of 0.7–0.9 and m(SiO₂)/m(Al₂O₃) of 3.0–4.0. This work provides a theoretical support for slag design in co-smelting process of SAC and CBES.