Abstract: Copper slag is very valuable secondary resource for metallurgy. The main mineral composition of the slag is fayalite, magnetite, copper matte and some amorphous vitreous.This experiment was based on thermodynamic analysis in which CO₂ and CO mixed gas was used to control the oxygen potential. The samples were reduced to elemental iron by the mixed gas when roasted at 600~900 ℃.At 1 000 ℃, the presence of FeO in the slag appeared due to the effects of oxygen in the mixed gas system.Fe₃O₄ was produced in the slag when the slag was calcined for 4 hat 1 100 ℃.The total iron content was 94.35 %, iron recovery rate reaching 90.8 %. The change of the fayalite phase at 600~1 100 ℃ was 2FeO·SiO₂→FeO·SiO₂→Fe→FeO→Fe₃O₄, and the recovery rate of iron rose with the temperature increasing in the roasting slag.It is beneficial for the formation of Fe₃O₄ when the roasting time is extended and the partial pressure ratio is increased within the proper range.