The experiment of electroslag remelting (ESR) was carried out under argon atmosphere by designing magnesium-containing slag system and adding strong deoxidizer to the slag pool continuously, and the possibility of increasing magnesium to electroslag ingot during ESR process was studied in detail. Magnesium content in electroslag ingot was analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES) and the effect of different magnesium content on the size, type and morphology of inclusions in electroslag ingots were analyzed in detail by ASPEX scanning electron microscopy. The research results showed that when the slag contains more than 20wt% MgO, even if the metal consumable electrode did not contain Mg, the MgO in slag pool still transferred Mg to the molten steel due to the strong reduction condition of deoxidizer. Under laboratory conditions, the Mg content in the slag ingot reached 0.0034wt%, 0.0039wt% and 0.0043wt% respectively when 55wt% CaF2–15wt% Al2O3–10wt% CaO–20wt% MgO slag, 65wt% CaF2–10wt% Al2O3–25wt% MgO slag and 51wt% CaF2–8wt% Al2O3–8wt% CaO–23wt% MgO–10wt% MgF2 slag were used for electroslag remelting. With the increase of Mg content in ESR ingots, the composition of inclusions in the electroslag ingot gradually changed from Al–Ca, Al–Mn–S, Al–Mg–Mn–S to Mg-containing inclusions, and the maximum content of Mg in inclusions was 98wt%. The number of inclusions decreased greatly, and the diameter of inclusions was smaller with the increase of Mg in the electroslag ingot. Especially, the maximum diameter of inclusions in electroslag ingots treated with Mg was less than 10 μm, and most of them were less than 5 μm. Compared with the electroslag ingot containing 0.0003wt% Mg, when the Mg content in the electroslag ingot increased to 0.0034wt%, the number of inclusions decreased from 357 to 31 pcs. The maximum diameter of inclusions decreased from 11.0 to 8.5 μm, and the average diameter decreased from 3.7 to 3.2 μm.