The calculation model of FeO activity for CaO-SiO2 -Al2O3 -MgO-FeO slag was formulated by the coexis- tence theory of slag structure. The effects of slag basicity， MgO and FeO content on FeO activity were analyzed at 1 400℃ . The results show that when the ternary basicity is 1.3， Al2O3 content is 12%， and FeO content is 2%， FeO activity increases with the increase of MgO content in slag. When the value of binary basicity， Al 2 O 3 and MgO content is 1.1， 12% and 10% respectively， FeO activity rises with increasing FeO content in slag. For the slag with 12% Al2O3 content， 10% MgO content and 2% FeO content， FeO activity rises rapidly when the binary basicity is increased. The above rules are consistent with that obtained by experiments， which illustrates the correctness for using the coexistence theory to calculate the FeO activity of CaO-SiO2 -Al2O3 -MgO-FeO slag.