Abstract: The longitudinal surface crack on thin slab is a serious quality defect at CSP line. Through analyzing the forming reasons, it is found that controlling the scrap quality, liquid steel composition, overheating degree in mould, powder quality, recycling water quality in the mould,keeping good condition of mould and segment,and the installing precision of submerged nozzle are effective to prevent the forming of longitudinal surface crack.Besides,maintain good match relationship between casting temperature and castiong speed is also effective to prevent the forming of longitudinal crack.After adopting the above measures,the rate of forming longitudinal crack is reduced to less than 0.4% stably at Zhujiang Steel.