The multiphase flows involving steel—slag—air phases in a two‐strand tundish during ladle change are investigated using mathematical modeling. Experimental results from water modeling are used to validate the simulated results. Tundish cover powder (TCP) and steel samples are taken from eight heats in one sequence to evaluate the effect of the multiphase flows during ladle change on the composition of the TCPs and steel cleanliness. Furthermore, the effect of the refilling time on the air entrainment and slag aperture during ladle change is analyzed and discussed. It is found that the strong turbulent flows in the tundish impact zone during ladle change promote the mix of the two‐layer cover powder, which consists of a basic TCP layer at the bottom and a top layer of rice hull balls. It results in a significant change in the composition of the TCPs and gradually deteriorates the steel cleanliness by the slag—steel reaction. Increasing the refilling time can effectively decrease the amount of entrained air and area of slag layer aperture in the tundish bath during ladle change. To reduce the steel reoxidation during ladle change, the refilling time should be larger than 184 s. Moreover, a new TCP is developed instead of the top layer of rice hull balls in industrial production. The use of the new TCP significantly improves the steel cleanliness in the tundish. The multiphase flow behavior in the tundish during ladle change not only easily causes the air entrainment and slag aperture, but also promotes the mix of the two‐layer cover powder. It has much influence on the steel cleanliness. Increasing the refilling time and controlling the composition of tundish cover powder can significantly improve the steel cleanliness in the tundish.