Abstract: Gleeble-3500 thermal simulation test, Factsage7.0 software, scanning electron microscope and infrared thermal imager are used to analyze the mechanism of high temperature thermoplastic and comer transverse crack formation of 0.125% C micro-alloying C36 ship plate steel 250 mm x 2 070 mm slab. The results show that the high temperature plasticity range of C36 ship plate steel is from 800°C to 1 200°C   , and the reduction of area of slab rate is 75.5% to 80.9% at 800 °C to 1 000 °C , and 87.8% to 95.0% at 1 050 °C to 1 200 °C. The large precipitation of the second phase NbC at 950 ~ 1 100 °C is the main reason to block the growth of recrystallized grains in C36 steel at this deformation temperature. The transverse crack at the comer of the cast slab of C36 ship plate steel is formed in the outer arc and in the intercrystalline brittle cracking. The formation of the crack may be related to the fact that the temperature (706 °C ) of the comer of the outer arc of the cast slab at the ninth stage of continuous casting of secondary cooling zone is close to the brittle temperature range and the static pressure is carried out. By increasing cast speed of C36 steel from 0.90 m/min to 0.95 m/min, cast slab out arc temperature be enhanced from 706 °C to 731 °C , the out arc comer crack yield rate decreases from 5.67% to 3.68%.