In order to implement continuous roll of steel plates, steel plates need to be jointed simply and reliably. CQ steel plates have advantages such as low impurity, composition uniformity, excellent surface quality and good mechanical properties. However, there always has fracture failure of the online welded joint for continuous cool-roll CQ plates. For solving this question, theoretical calculation of weldability, fracture morphology and metallurgical structure were analyzed. The results show that CQ steel plates have good weldability according to the chemical composition, and the microstructure of welding seam are mainly ferrite and a small amount of lath martensite.While grain growth in HAZ is serious, with average grain size 15μm. A lot of impurities and gas cavities in welding seam are the main source of cracks. In addition, stress concentration caused by coarse grain zone in HAZ is one of the reasons for causing cracks. The microhardness was test and the results show that the hardness decrease from weld center to the base metal and there is a minimal value where is softened zone.