High-aluminium steels contain a significant amount of aluminium. The reaction between Al in the liquid steel and SiO₂ in lime-silica-based mould powders during the continuous casting process of high Al steel causes chemical compositional changes in the mould powders, subsequently affecting the surface quality of slabs. In order to solve the aforementioned problem, lime-alumina based mould powders have been developed which can lead to an increase in the surface quality of cast slabs by inhibiting molten steel/slag interaction. However, the mould slag tends to crystallise easily, which leads to a deterioration of the mould lubrication. In this study, viscosity of lime-alumina-based mould powders with the addition of B₂O₃ and the effects of increasing the CaO/Al₂O₃ ratio have been observed through IPT (Inclined Plate Test) and rotational viscometry. Additionally, FactSage software and empirical models were used to calculate viscosity of the mould powders. The results show that the viscosity of mould powders decreases dramatically while CaO/Al₂O₃ ratio changed from 1 to 1.5. The viscosity of designed mould powder shows that with the increase in CaO/Al₂O₃ ratio ranged from 1.5 to 2.5, viscosity slightly decreases, then when the CaO/Al₂O₃ ratio over 2.5, viscosity nearly levels off.