In order to understand the causes of cracking and damage of magnesium slag dam in tundish，the physical properties，phase composition，thermal shock resistance and slag resistance of castable preforms for slag dam at room temperature were detected and analyzed by X-ray diffraction，scanning electron microscope and other detection methods.The results show that molten steel and slag infiltrate along the pores and cracks of the slag dam of the tundish，and forsterite and slag react in the erosion and infiltration layer to form low melting point monticellite or magnesium rhodonite，etc.At the same time，the molten slag infiltrating in the refractory material causes the composition of the infiltration layer and the original layer to be different，and the thermal expansion and cold contraction are inconsistent，resulting in separation and cracking，which generates a large number of cracks and intensifies the cracking damage of the slag dam.