Abstract: Based on the principle of Gibbs free energy minimization, the thermodynamic model of the freeze slag inside reaction shaft of copper flash smelting furnace is established by using the freeze slag as the fourth phase during copper flash smelting process. Results show that the freeze slag with high content of Fe₃O₄ can be generated when the oxygen potential in reaction shaft is high enough, but the producing rate of freeze slag is only about 1.0% of smelting feed. Compared with practical measured data, the relative errors of the percent content of Fe₃O₄, SiO₂, FeO and Cu₂O in freeze slag are 9.24%, 9.93%, 8.10% and 6.73% respectively. It verifies that the thermodynamic model is feasible, which lay the foundation for the study of thermodynamic of freeze slag inside reaction shaft.