The advantages of the electroslag remelting (ESR) process as to cleanness and homogeneity of the ingot structure are well known. As to homogeneity of composition, the control of titanium in stainless steel with high titanium and low aluminium contents during the ESR process has not been resolved well so far. The current work focuses on designing appropriate slag for controlling titanium content during 1Cr21Ni5Ti remelting based on the interaction of the slag/metal interface. Several kinds of slag containing different CaO contents combined with steel samples of 1Cr21Ni5Ti were employed to investigate the effect of slag on titanium content in an electrical resistance furnace, and metal samples were taken at different times for investigating the change of titanium and aluminium contents in steel. The results show that slag with low CaO content at low temperature has excellent capacity for avoiding loss of titanium content caused by its reaction with the alumina in slag, especially in case of remelting of stainless steel with high titanium and low aluminium contents. The CaO in slag has great effect on the activities of TiO₂ and Al₂O₃, and thermodynamic analysis was applied to investigate the CaO behaviour based on the ion and molecule coexistence theory of slag. The thermodynamic analysis results show that the increases as CaO content in slag decreases, which is better for preventing loss of titanium caused by its reaction with the alumina in the ESR slag. The slag with low CaO content at low temperature is suitable for ESR of 1Cr21Ni5Ti.