Abstract: From the composition, macroscopic morphology, micro-fracture, precipitates and solidification structure of the billet, the crack defects in the core of 27SiMn steel round billet were detected and analyzed. The results show that at the end of solidification of 27SiMn round billet, the defects of shrinkage porosity and shrinkage holes are formed due to insufficient feeding at the core; S, Mn and other elements are enriched and precipitated on the free surface of the metal to form ovate dendrites, which reduces the continuity of the microstructure and produce cracks. The stress release during subsequent process makes the cracks further expand and form star cracks. By controlling the casting temperature, reducing the water volume in secondary cooling zone and pulling speed, the star crack defects in the core of 27SiMn round billet are effectively eliminated,and the yield of the cotinuous casting billet with large section is improved.