Abstract: Investigating the reaction mechanism between slag and 9CrMoCoB steel is important to develop the proper slag and produce qualified ingots in the electroslag remelting (ESR) process. Equilibrium reaction experiments between molten 9CrMoCoB steel and the slags of 55wt%CaF₂–20wt%CaO–3wt%MgO–22wt%Al₂O₃–xwt%B₂O₃ (x = 0.0, 0.5, 1.0, 1.5, 2.0, 3.0) were conducted. The reaction mechanisms between molten 9CrMoCoB steel and the slags with different B₂O₃ contents were deduced based on the composition of the steel and slag samples at different reaction times. Results show that B content in the steel can be controlled within the target range when the B₂O₃ content is 0.5wt% and the FeO content ranges from 0.18wt% to 0.22wt% in the slag. When the B₂O₃ content is ≥1wt%, the reaction between Si and B₂O₃ leads to the increase of the B content of steel. The additions of SiO₂ and B₂O₃ to the slag should accord to the mass ratio of B/Si in the electrode, and SiO₂ addition inhibits the reaction between Si and Al₂O₃.