To illuminate the corrosion behavior of MgO-based refractories under electromagnetic field (EMF), herein, the slag corro-sion and penetration resistance of MgO–MgAl2O4, MgO–CaO, and MgO–C refractories were investigated using the rotary immersion slag resistance test at 1873 K for 1 h. The results showed that the order of the good slag resistance of as-tested refractories was MgO–C > MgO–CaO > MgO–MgAl2O4. The EMF accelerated the corrosion and penetration of slag to the refractories, which caused the molten slag to be easier into the refractories by natural convection and Marangoni effect. In addition, the MgO–C refractories did not show an overwhelming advantage in slag resistance because EMF impeded the formation of the dense protection layer. Consequently, in view of the present results, the MgO–C refractories are still the most promising slag line material for refining furnace among MgO–MgAl2O4, MgO–CaO, and MgO–C refractories.