Abstract: The FeO-SiO₂-Al₂O₃-CaO system is a common slag type in the matte smelting of copper concentrate and the reduction smelting of tin concentrate. The transport properties of the melt have a crucial influence on chemical reaction kinetics, slag-metal separation efficiency, and the content of valuable metals in the slag. In this paper, a comparative study on the local structure and transport properties of copper slag and tin slag melts in the FeO-SiO₂-Al₂O₃-CaO system was carried out using molecular dynamics (MD) simulation, aiming to reveal the essential mechanism for the different effects of CaO and FeO contents on the transport properties of the melts. The results show thatSiO₄^4- tetrahedra andAlO₄^5- tetrahedra are the main structural units of the network skeleton, andSiO₄^4- tetrahedra are more stable thanAlO₄^5- tetrahedra. In the copper slag system, as the mass fraction of CaO increases from 3% to 18%, non-bridging oxygen and free oxygen transform into bridging oxygen, structure unit Q₀、Q₁ and Q₂ transform into Q₃ and Q₄, the degree of structural complexity (DSC) and degree of polymerization (DOP) of the system increase, and the viscosity increases from 0.21 Pa·s to 0.30 Pa·s. In the tin slag system, as the mass fraction of FeO increases from 3% to 18%, bridging oxygen transforms into non-bridging oxygen and free oxygen, Q₃ and Q₄ transform into Q₀、Q₁ and Q₂, the DSC and DOP of the system decrease, and the viscosity decreases from 0.66 Pa·s to 0.37 Pa·s. At the same content of CaO in copper slag and FeO in tin slag, the DSC and DOP values of copper slag are lower than those of tin slag, and the viscosity of copper slag is also lower than that of tin slag. Fe²⁺ and Ca²⁺ act as both network modifiers and charge balancers in the system; the network modification ability of Fe²⁺ is stronger than that of Ca²⁺, while the charge compensation ability of Ca²⁺ is stronger than that of Fe²⁺. The efficiency of FeO to reduce the viscosity is about three times that of CaO. The results provide a theoretical and technical basis for the efficient utilization of copper and tin resources.