Abstract: A 3D mathematical model was proposed to investigate the molten steel–slag–air multiphase flow in a two-strand slab continuous casting (CC) tundish during ladle change. The study focused on the exposure of the molten steel and the subsequent reoxidation occurrence. The exposure of the molten steel was calculated using the coupled realizable k–ε model and volume of fluid (VOF) model. The diffusion of dissolved oxygen was determined by solving the user-defined scalar (UDS) equation. Moreover, the user-defined function (UDF) was used to describe the source term in the UDS equation and determine the oxidation rate and oxidation position. The effect of the refilling speed on the molten steel exposure and dissolved oxygen content was also discussed. Increasing the refilling speed during ladle change reduced the refilling time and the exposure duration of the molten steel. However, the elevated refilling speed enlarged the slag eyes and increased the average dissolved oxygen content within the tundish, thereby exacerbating the reoxidation phenomenon. In addition, the time required for the molten steel with a high dissolved oxygen content to exit the tundish varied with the refilling speed. When the inlet speed was 3.0 m·s⁻¹ during ladle change, the molten steel with a high dissolved oxygen content exited the outlet in a short period, reaching a maximum dissolved oxygen content of 0.000525wt%. Conversely, when the inlet speed was 1.8 m·s⁻¹, the maximum dissolved oxygen content was 0.000382wt%. The refilling speed during the ladle change process must be appropriately decreased to minimize reoxidation effects and enhance the steel product quality.