Abstract: Based on Fluent software, a three-dimensional model of 150 mm×150 mm mold was built. The variation in flow field, temperature field and solute distribution in the mold were calculated and the macro-segregation in the secondary cooling zone was simulated. It was found that the solute at the corner of the mold solidified faster than that on the surface as heat transfer of the former is two-dimensional not one-dimensional. The molten steel at the corner of the mold recirculated, and that at the meniscus of the mold refluxed slightly. Under the influence of recirculation and solidification, carbon was redistributed in the mold so that there appeared positive carbon segregation at the upper part of the mold and negative one in the recirculation channel. It was also found that there was annular negative carbon segregation during the entire cooling of the casting billet as a result of the difference in solid-liquid phase diffusion coefficient.