Based on the principle of similarity, a study has been conducted on the effects of the original design and the optimized design upon the flow field in tundishes. With the stimulus-response approach adopted, the average residence time distribution (RTD) curves of the tundish fluid flow can be obtained and the effect of the two sets of flow control devices on the fluid flow behavior in tundishes can be worked out, thus optimizing the control on the flow field in tundishes. The results show that the plug flow volume has been increased by 42.44%, the average residence time of steel liquid flow in the tundish has been extended by approximately 500 s, and the homogeneity of the molten steel flow in each outlet has been improved, thus facilitating the removal of the floating inclusions.