The secondary dendritic arm spacing(SDAS)of free cutting steel 38MnVS during continuous casting process was simulated based on a Mixed Lagrangian and Eulerian Method(MiLE Method)and SDAS model.The simulation results are basically in agreement with measured ones.The effect of composition,superheat and casting speed on SDAS are studied,and the relationship between SDAS and interdendritic segregation is discussed.The results show that SDAS increases with increasing carbon and silicon content,and decreases with increasing manganese and sulfur content.The increase of superheat and casting speed also makes SDAS increase.The permeability of columnar mushy zone which is parallel to the primary dendritic arms is calculated based on Carman-Kozeny relationship,and the increase of SDAS makes the permeability increase,which exacerbates interdendritic segregation of columnar crystal zone. The secondary dendritic arm spacing (SDAS) of free cutting steel 38MnVS during continuous casting process was simulated based on a Mixed Lagrangian and Eulerian Method (MiLE Method) and SDAS model. The simulation results are basically in agreement with the measured ones. The effect of composition , superheat and casting speed on SDAS are studied, and the relationship between SDAS and interdendritic segregation is discussed. that results show that SDAS increases with increasing carbon and silicon content, and decreases with increasing manganese and sulfur content. increase in superheat and casting speed also makes SDAS increase.The permeability of columnar mushy zone which is parallel to the primary dendritic arms is calculated based on Carman-Kozeny relationship, and the increase of SDAS makes the permeability increase, which exacerbates interdendritic segregation of columnar crystal zone.