A three-dimensional mathematical model has been developed to predict steady flow field, pressure distribution inside the shroud and efficiency of inclusion removal in tundish after argon gas injection at the shroud. Effects of the different opening of the slide gate and gas injection on velocity and pressure have been analyzed. The results show that the molten steel velocity gradually decreases and the pressure increases as the opening of the slide plane becomes larger. The velocity increases and the pressure decreases after gas injection. Bubble adhesion plays an important role in inclusion removal. Smaller bubbles and larger inclusions are favorable for inclusions removal. The larger flow rate of gas is, the greater number of bubbles generated is. Thereby the amount of removed inclusion increases. But local boiling will occur in the case of larger rate of gas, so the flow rate of gas should be strictly controlled in practice.