Abstract: The hot ductility of SS400 steel continuous casting slabs was tested by Gleeble-3500 thermal simulation machine, and the fracture surface morphology of the tensile specimens was observed using scanning electron microscopy. The results show that the third brittle temperature range of the steel was 725 - 825 ℃, and at the brittle temperature range, the reduction percentage of the steel was only about 40%. By controlling the cooling rate of the secondary cooling zone and raising the billet straightening temperature above 850 ℃ to avoid the third brittle temperature range, it could reduce the generation of surface cracks of the cast slab.