Abstract: The formation causes of longitudinal cracks on the surface of double-roll thin strip continuous casting samples were analyzed by optical digital microscope（OM）, scanning electron microscope（SEM）, and energy spectrum（EDS） technology. The results show that the surface longitudinal cracks are high-temperature thermal cracks formed along the grain boundary during the solidification process. There are dendritic FeO inclusions on two walls of the large surface cracks and the small cracks. The analysis believes that under the action of rolling tensile stress, the liquid FeO that exists at the grain boundary below the melting point of the low carbon steel will weaken the connection between the grains and become the source of cracks under the action of external tensile stress, which is one of the causes of the longitudinal direction thermal cracking.