According to the defects analysis of sulfur ensuring steel strands（300 mm× 360 mm）， it is considered that the inclusions as FeS， MnS easily precipitates out along the grain boundary during solidification process. This leads to reducing the ability for resistance stress at the grain boundary， so it is bring subsurface crack and intermedi- ate crack on applied stress. The reduction of mould coolling water and improvement of mould powder capability can eliminate the subsurface crack and intermediate crack of the bloom casting billet.