Abstract: Carbonate method was applied to the determination of active MgO in steel slag. The feasibility and accuracy of this mehod was studied. It was found that MgO and Mg(OH)2 were converted to magnesium ion by reacting with CO2 in water,and then the magnesium ion was detected using EDTA standard solution. The reaction yields were higher than 97%. The reaction time of CO2 with raw steel slag should be more than 105 min,and the mass fraction of active MgO of Xinyu steel slag was 1.44%. The mass fraction of Mg(OH)2 of steel slag after autoclave test was 1.86%,which was in agreement with the value of active MgO in raw steel slag,and the relative error was lower than 1%.