Abstract: During the molten steel filling process in the tundish for the first heat of continuous casting, unreasonable design of flow control devices can easily lead to secondary oxidation of molten steel, erosion of refractory materials, and significant impacts on steel cleanliness, though few related studies exist. This study investigates an 8 t single-strand small rectangular tundish in an industrial plant. Physical and numerical simulations were combined to analyze the molten steel flow behavior in the prototype tundish, and a mathematical model was used to calculate the degree of secondary oxidation. To address deficiencies in the prototype, the original baffle with flow holes was replaced with an optimized dam-weir configuration. The effects of dam/weir heights and positions on molten steel flow were systematically studied. The results show that the prototype tundish exhibited severe secondary oxidation and refractory erosion near the front wall during initial filling, along with short actual residence time. The dam-weir configuration demonstrated effective suppression of secondary oxidation. The optimal parameters were determined as follows: weir height 180 mm from the tundish bottom, dam height 300 mm, and 370 mm distance between the stopper rod center and dam. Industrial implementation confirmed significant improvements: total oxygen content decreased, MgO·Al₂O₃ inclusions were notably reduced, front-wall refractory erosion weakened markedly, and dead zone proportion decreased to 1.2% after optimization.