Abstract: 3xxx series aluminum alloys were produced by continuous casting and rolling with copper roll sleeves. By analyzing the characteristics of the dendrite structure with dark and bright striations in cast-rolled plates, and the dynamic changes of the crystallization front in the process of casting and rolling, the reasons for the occurrence of these striations on the plate surface during such process were studied, with control measures proposed. The results showed that the repeated contact and separation between the solidified shell and the roll surface caused discontinuous cooling of the melt by the roll. This led to the discontinuous crystallization on the surface layer of the strip billet, and finally formed the transverse defect on it. The formation of bright crystals even aggravated the severity of the transverse defect. When the casting rolling speed increased to a certain value, the approaching speed (V_a) of the roll towards the solidified shell was greater than or equal to its shrinkage speed (V_s). There was no air gap between the initial solidified shell and the roll surface. At the same time, the formation of bright crystals was restrained, which could significantly reduce or even eliminate the transverse defect of the casting plates.