Investigating the reaction mechanism between slag and 9CrMoCoB steel is important to develop the proper slag and produce quali-fied 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 elec-trode,and SiO2 addition inhibits the reaction between Si and Al2O3.