The invention relates to a slag system for electroslag remelting of B-containing type 9Cr heat resistant steel. The slag system comprises the following components in percentage by mass: 50-60% of CaF2, 15-25% of Al2O3, 15-25% of CaO, 3-8% of MgO, 0.5-3% of B2O3, and equal to or less than 1% of impurities, wherein the sum of the mass percentage of the components is 100%; and the impurities compriseinstable oxide SiO2 accounting for equal to or less than 0.8% of the mass of the slag system. The slag system can be used for electroslag remelting to produce B-containing type 9Cr heat resistant steel, such as COST-FB2 steel or CB2 steel; application of the slag system can reduce burning loss of the B element, so that the B content is uniformly distributed in the cast ingot height direction, andthe uniformity of the alloy structure is ensured to further improve the high temperature creep property of alloy; and in addition, the lower melting point of the slag system enables an electroslag ingot to solidify and form easily, so that the surface defect of the electroslag ingot can be effectively reduced or eliminated. The melting point, viscosity and other physical property parameters of the slag system can meet the requirement of electroslag remelting and smelting of B-containing type 9Cr heat resistant steel.