<p>Behaviors of slag infiltration and shell initial solidification near the meniscus are extremely critical to the surface quality of the ultra-large beam blank (ULBB). A three-dimensional (3D) numerical model with the consideration of the factors such as the steel solidification, heat transfer, steel–slag interfacial tension, multiphase flow and mold oscillation was developed. With this model, the transient flow behavior of the molten steel in the mold, the meniscus behaviors of the slag infiltration and shell initial solidification during the ULBB continuous casting (ULBBCC) were investigated, and the interaction between them was revealed. The results show that the steel top surfaces in the areas of the web, the outer corner and tip of the flange are affected by the complex upper reflow of the ULBB mold (ULBBM), leading to violent fluctuations and a high steel level. However, the areas of the inner corner and bevel of the flange, the fillet and the center of the narrow face have a low steel level. The total slag thickness in the slag channel near the meniscus is positively correlated with the slag rim thickness except for the inner and outer corners of the flange. At 10 mm below the initial steel–slag interface, the total slag thicknesses at the inner and outer corners are considerably thick and thicker than other areas, with thicknesses of about 3.4 and 3.5 mm, respectively. The fluctuation behavior of the complex steel–slag interface will disturb the slag infiltration to form the non-uniform slag film, which hinders the uniform inflow of liquid slag, and thus promotes the non-uniform growth of the shell.</p>