The longitudinal cracks exist at the surface of heated 150 mm x 150 mm continuous casting billet of steel 55(/% ：0.52 ~0.60C

0.17 ~0.37Si

0.50 ~0.80Mn

≤ 0・035P

≤0.035S). the subsurface crack defects of eating billet are analyzed by metallographic microscope. The result indicates that the defect is caused by the strong non-nomogneous secondary cooling and excessive harmful element Pb enriched at the grain boundaries. The optimization process of the spray system of the second cooling and reducing the harmful element Pb in steel is carried out. It improves the cooling uniformity and grain boundary strength of casting billet. The result shows the defects of casting billet have been effective controlled. The qualification rate of the first inspection of rolled products increases from 45% to 93%.