Secondary copper smelting is an effective means of treating waste resources. During the smelting process, the viscous behavior of the smelting slags is essential for smooth operation. Therefore, the effects of Fe/SiO₂ ratio and Fe₂O₃ contents on the viscous behavior of the FeO−Fe₂O₃−SiO₂−8 wt%CaO−3 wt%MgO−3 wt%Al₂O₃ slag system were investigated. The slag viscosity and activation energy for viscous flow decrease with increasing Fe/SiO₂ from 0.8 to 1.2, and increase as the Fe₂O₃ content increases from 4 wt% to 16 wt% at Fe/SiO₂ ratio of 1.2. However, under the conditions of Fe/SiO₂ of 0.8 and 1.0, the viscosity and activation energy for viscous flow show a minimum value at Fe₂O₃ content of 12 wt%. Fe₂O₃ exhibits amphoteric properties. In addition, the increase in Fe₂O₃ content raises the breaking temperature of the slag, while the Fe/SiO₂ ratio has the opposite effect. Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy show that increases in Fe/SiO₂ ratio lead to simplification of the silicate network structure, while increases in Fe₂O₃ content improves the formability of the network. This study provides theoretical support for the related research and application of secondary copper smelting.