Abstract: The wetting characteristic of mold flux is an important factor affecting the surface quality of low-density steel billets. By using a high-temperature contact angle measuring instrument in combination with interfacial structure characterization, the effects of（CaO+BaO）/Al₂O₃ mass ratio and substitution of B₂O₃ for SiO₂ on the wetting characteristic between CaO-Al₂O₃-based mold flux and low-density steel are studied. Results show that as the（CaO+BaO）/Al₂O₃ mass ratio increases from 0.87 to 1.59 and B₂O₃ gradually replaces SiO₂, the initial temperature at which the contact angle begins to rapidly reduce decreases from 1 480 ℃ to 1 330 ℃, 1 170 ℃, and first decreases from 1 300 ℃ to 1 170 ℃ and then increases to 1 180 ℃, respectively. The evolution behavior of contact angle and adhesion work is similar to the change in temperature. As the（CaO+BaO）/Al₂O₃ mass ratio increases, the thickness of element diffusion layer at the steel-slag interface increases from 1 μm to 5 μm. As B₂O₃ replaces SiO₂, the thickness of element diffusion layer at the interface first decreases from 9 μm to 5 μm and then increases to 15 μm. Mn diffuses and transfers from the steel matrix to the interface, and its content gradually decreases. Al diffuses and transfers from the steel matrix to the interface, and due to the high Al content in the slag at the interface, its content shows an increasing trend. The change of Si content is not obvious. Based on this, the mass ratio of（CaO+BaO）/Al₂O₃ is controlled at around 1.59, and the contents of B₂O₃ and SiO₂ are controlled at 5%, which are conducive to maintaining the low reactivity of mold flux and improving its wetting characteristics with low-density steel.