Abstract: The mechanism of slag entrapment at the overflow weir in a tundish was studied by the combined methods of numerical simulation, water modeling experiment and industrial test. The results show that liquid steel between the dam and weir is at the risk of being exposed to air and being oxidized when its level (surface) is close to the weir top. At the same time, steel-slag emulsification and slag entrapment at the weir outlet area may happen. The possibility of slag entrapment is higher at the triangle area from the weir to the tundish wall. The liquid steel flow over the weir will erode the weir, and also shorten the life of the tundish. In addition, the entrapped slag is easier to enter the mould. This result is confirmed and verified by water modeling experiment and industrial test.