Based on the calculation of flow flied and temperature field, this article has made a numerical simulation on a slab casting mold that has a section of 1 780 mm×225 mm. In this process, three cases with different nozzles and a case without nozzle are compared to research the effect of flow field to solidification shell. It could be seen that, the maximum shell are about 45 mm at the corner and 24 mm at the center of wide face and narrow face. However, all of these are about 1-2 mm thinner than the case without nozzle. Besides, the flow of liquid steel would form a 2.5 mm sunken, which is 300 mm away from corner and 400 mm below meniscus. The molten steel would scour the whole narrow face, especially at the center. The pointed-bottom nozzle has the smallest shell and recessed-bottom has the largest shell.