On the basis of the composition and liquidus temperature characteristics of bainitic frog steel, the phase diagram of the CaF2-Al2O3-CaO-MgO slag system, as well as physical parameters such as the melting point, viscosity, and conductivity, was calculated via FactSage 7.3 thermodynamic software. Furthermore, a new specialized slag system for bainitic frog steel was optimized. The differences in the metallurgical quality, composition, and microstructure of bainitic frog steel during electroslag remelting between the traditional binary slag system containing 70 wt.% CaF2-30 wt.% Al2O3 (slag 1#) and the designed slag system containing 47.5 wt.% CaF2-21.5 wt.% Al2O3-28 wt.% CaO-3 wt.% MgO (slag 2#) were investigated in a 10 kg electroslag remelting furnace. Compared with that of the traditional binary slag system, the desulfurization rate of bainite frog steel smelted by designing slag-based electroslag remelting is greater, reaching 90.91%. However, there is a certain degree of burning loss of silicon in steel, and the increase in hydrogen content is greater during electroslag remelting by slag 2#. The microstructure of the electroslag ingots smelted by the two slag systems is relatively good. The designing slag based remelting electric slag ingot has better surface quality.