Q345B (containg Nb) is the sort of high strength low alloy (HSLA) steel which is sensitive to cracks. We analyzed the influences of C, Nb and the secondary cooling water flow rate on the cracks in the casting blank and the reasons for crack formation. Based on those results, we optimized the secondary cooling water distribution system in the process of continuous casting through industry tests and proposed a low cooling water distribution system and uniform cooling. The results show that when the temperature of a casting blank is above 950 ℃, it can prevent the cracks effectively, reduce the generation probability of cracks, and enhance the quality of casting blanks by 80%.