Abstract: Aiming at the problem that the high content of magnesium in ferronickel slag produced by electric furnace smelting laterite nickel minerals leads to the inability of slag to be used in large quantities and the waste of magnesium resources, the technical idea of chlorination of MgO in volatile slag with CaCl₂ under the condition of carbon coexistence was proposed. The effects of chlorination temperature, holding time, carbon and calcium chloride addition on the volatilization rate of magnesium in slag were studied. The results showed that the volatilization rate of magnesium in ferronickel slag could reach 67.22% and the content of MgO in the slag after chlorination was reduced to 8.21% when n（C）/n（Mg） = 1.25, n(CaCl₂) / n(Mg) = 1, and holding at 1 300 ℃ for 120 min. The extraction process of magnesium from chlorination slag with CaCl₂ under the condition of carbon coexistence was that O₂ and acidic oxides in the slag made CaCl₂ decompose to Cl₂, and then it reacted with MgO to form MgCl₂. The role of carbon was to reduce the reaction temperature of Cl₂ and MgO. With the chlorination process, the main phase in the slag changed from Mg₂SiO₄ to CaMgSi₂O₆, and the melting temperature of the slag decreased from the initial 1 400 °C to 1 300 °C.