Abstract: The compound leaching slag of copper anode slime is the product of compound leaching of arsenic, antimony and bismuth from copper anode slime, and Selenium (Se) is selectively separated by sulfated roasting. In view of the unclear reaction process of the selenium-containing substances in the selective separation process, relevant thermodynamic data in Factsage and Equilib equilibrium calculation module were used to analyze the ΔG^θ of selenium-containing substances reacting at different temperatures and reaction process balance, to clarify the law of phase change in reaction process. The results show that at low temperature Cu₂Se reacts with H₂SO₄ to generate solid Se, CuSO₄·H₂O, SO₂ and H₂SO₄ absorbs water to form H₂SO₄·H₂O. Solid Se then melts into liquid Se and simultaneously reacts with H₂SO₄·H₂O to form SeO₂, SO₂ and H₂O. At high temperature, CuSO₄·H₂O will be dehydrated to produce CuSO₄ and H₂O, and CuSO₄ will be further decomposed to produce CuSO₄·CuO and SO₃. Ag₂Se reacts with H₂SO₄ to generate solid Se, Ag₂SO₄, SO₂ and H₂SO₄ absorbs water to form H₂SO₄·H₂O, and then solid Se melts into liquid Se and reacts with H₂SO₄·H₂O to form SeO₂, SO₂ and H₂O. In addition, Ag₂SO₄ will change form at high temperature, and Se reacts with H₂SO₄ to generate SeO₂, SO₂ and H₂O. Therefore, the Se evaporation rates of compound leaching slag of copper anode slime at different roasting temperatures, with different sulfuric acid dosages and for different roasting time were analyzed. Under the roasting condition of temperature at 500 ℃, the added quantity of H₂SO₄ of 0.2 mL/g, the warming rate 10 ℃/min and the reacting time 30 min, Se evaporation rate reached 95.4%.