Abstract: The sulfur behavior characteristic in the process of copper slags smelting reduction is studied. When slag basicity increases from 0.8 to 1.4, the slag sulfur capacity and desulphurizing capacity increases and the molten iron sulfur content decreases from 0.6% to 0.13%. Desulfurization reaction of molten iron is endothermic, and the molten iron sulfur content decreases from 0.13% to 0.089% with the holding temperature increases from 1773 K to 1823 K. The theoretical sulfur distribution ratio between molten slag and iron is higher than the value under the test time. The desulfurization rates of the molten iron increase with the holding time. Sulfur content of the molten iron decreases to 0.11% after being treated under the holding temperature of 1823 K for 40 minutes with slag basicity of 1.4. The sulfur content in the generated iron is still high and the further desulphurization pretreatment is needed for steelmaking.